Hi thread,
As everyone reading this is probably aware already,
Raikou and Registeel both rose to OU and therefore are no longer allowed in standard UUBL. This is of course not without consequence on the metagame as they were both almost universally regarded as top 3 Pokemon in the tier alongside Dragonite. There will not be a Viability Ranking update just now as the tier shift just happened, so that will probably wait until after we get some tournaments play in ADVLTL. Nonetheless, I will add onto Erfder's post by reharsing and sharing some of my own predictions regarding the impact on the viability of some Pokemon.
Raikou has always been, among other roles, far and away the best Electric-type in a metagame with a lot of bulky Water-types, notably Vaporeon which used to be really difficult to force out through sheer immediate offensive pressure alone. However, interestingly, Lanturn initially owed its attention to Raikou because it was one of its few non-niche counterplays available, and then came Regice which further buffed Lanturn due to its unique profile of being a special attacker with a resistance to boltbeam. At first I was uncertain of Lanturn's future with the Pokemon the Pokemon that gave it its initial popularity gone, but the Raikou is Kill tour instead made Lanturn more appealing by ever since it was considered the second best Electric-type that did what Raikou was often tasked with : scarring away spikers like Qwilfish and Omastar in the lead position and switch into waters like Vap and Slowbro more easily that the rest of the frailer and non Water resistant UU electrics.
Another interesting to note regarding Lanturn is its SubToxic set, which was seen by some as its best set for its high HP Substitute that allowed it to challenge almost every special wall and notably dominate any non Rest Regice. Key word being "almost" because it was still shut down by Registeel which very commonly carried Earthquake and was immune to Toxic, but now Registeel is gone as well and thus Lanturn loses one of its biggest counters. SubToxic Lanturn also used to suffer from a 4MSS because it had to choose between Ice Beam and Surf, and losing Surf meant losing the ability to break a +1 Raikou's substitute and worsening its Steelix matchups. Raikou being banned in the aforementioned Raikou is Kill tournament naturally lifted the first concerned and Steelix was made much less relevant due to that very ban. Steelix may be less impacted now but those kind of development still likely favor SubToxic Lanturn. So all in all, Lanturn seems to be the Pokemon that benefited from every UUBL tier shift.
And then there are other UU Electrics that have a better chance at shining with Raikou gone, Ampharos being one of them as it is retains the same Special Attack as Raikou and as some useful tools of its own, most notably Heal Bell. It also has good enough special bulk so it can afford to take some Surf from non offensive Vaporeon, Omastar, and Slowbro, which is something that the faster Electric-types like Manectric, Raichu, Electrode, and Electabuzz wish they could.
Similarly to how Raikou was the best Electric-type, Registeel was regarded as the best Steel-type by a long shot. This trait was, as it often is in a Pokemon metagame, very significant, as the UUBL Pokemon list is littered with strong Normal and Psychic-types. A big thing that made Registeel the most popular Steel-type is its massive special bulk that made it much sturdier against Psychic-types even when factoring the potential Fire-coverage and that was more than sufficient to take on other special threats on raw bulk alone, including Raikou and offensive waters. On that end, Scizor cannot compete, but it is still a sturdier Normal check overall due to its neutrality and access to Morning Sun, and is special bulk is more than good enough to take on special threat with resisted STAB like Regice that almost never runs HP Fire. Now that Registeel is gone, other Pokemon like Alakazam and Espeon may be more likely to drop their Fire coverage and Scizor might just benefit from this.
As for Steelix, its relationship with Registeel was interesting : if you could draw a circle containing offensive threats that Steelix keeps in check (which essentially includes Raikou and some normal-types), you could draw a bigger circle on top of this one and add a ton of other Pokemon (Regice, Alakazam, non Sub Vaporeon, Lanturn, Kingdra, Jynx, etc.) to list what Registeel kept in check. That is to say, Steelix suffered a ton from its competition with Registeel, that could also, admittedly by a lesser margin, handle what Steelix was tasked to deal with, and much more. On the other hand, Steelix was also useful at thwarting its biggest competitor, not caring about Toxic, Thunder Wave, or the occasionnal Explosion, phazing any Curse attempts, and threatening it with a STAB Earthquake. So in that sense, Registeel was ironically both a huge competitor and another reason to run Steelix. Unlike Scizor, Steelix can hardly hope to be slotted as counterplay to Psychic-types in a Registeel-less meta as even if they don't pack Fire coverage, the threat of HP Water is still very real with Houndoom around, and the one special threat it could (semi) reliably handle in Raikou is gone too. Steelix's future is still a bit uncertain to me, but its ability to phaze (key against the likes of Miltank), Toxic and Thunder Wave immunity, boom, and STAB Earthquake that make it not Spikes bait for Qwil and Oma remain valuable traits among Normal resistances.
Raikou was the number one special threat a player wanted to handle with their special check, and Registeel was the most common special wall that exploited a lot of passive Pokemon that relied on Toxic to damage opposing bulky Pokemon. With both gone, the number of prime candidates for the role could increase, because a good number of Pokemon with a seemingly fit profile to handle special threat did poorly against Raikou, gave free entry to Registeel, or both. Regice was a capable Raikou answer with relatively unconventional tools like Psych Up and the prime special tank for offense. Its usage dropped by quite a few ranks in Raikou is Kill tournament which was less offensively inclined than the Raikou UUBL, at least going by that particular tournament usage stats. Perhaps story will repeat itself with Raikou gone for a (presumably) longer future, but Registeel leaving too, the balance could instead swing in Regice's favor, since one of its hardest counters just left.
Articuno is another Pokemon that got a lot a popularity in a Raikouless metagame. Its Electric weakness is nowhere near as big of a deal now, and its access to Heal Bell, ease to fit phazing moves like Roar, Spikes immunity, and to a lesser extent Pressure gives it a lot of merit here. Registeel leaving is another boon for Articuno, no longer being left in a helpless state or reliant on a lucky freeze against the golem without Spikes and Roar. There also have been some experiments with more offensive Articuno and moves like HP Grass and even Extrasensory, in order to make it more threatening to Pokemon used to exploit it like Qwilfish, Omastar, and Hariyama, and once again Registeel leaving might make offensive Articuno easier to run.
Umbreon is notorious for its utter passivity, which notably plagued it against SubCM Raikou and Registeel that could farm Leftovers recovery and threaten it with Toxic without having to fear Synchronize. Umbreon's passivity remains a big issue, but its capabilities as a special wall might be better than ever if Alakazam becomes the number one special threat of note. Curse Pass sets have also been toyed with.
And then there are other specially bulky Pokemon like Kangaskhan which were flimsy against Raikou as they lack the special bulk to live more than one hit from a boosted Raikou after Spikes and/or the offensive power to kill the deceptively phsyically bulky Raikou before that happens. Now, most of the special threats are either much frailer (Alakazam) or can't reach the same level of power as Raikou without some CM BP shenanigans (Kingdra), which should make handling special threats in UUBL an all around easier job for Pokemon like Kangaskhan. Speaking of which, Kanga might have even more room with its moveslots now that btoh Raikou and Registeel are gone and Earthquake easier to drop as a result.
One of the things I noticed soon after one of my first few games of UUBL back in 2018 is that there wasn't a single Electric-type, and then Raikou dropped the following year, and then it rose back to OU 7 years later. Bulky Waters in Raikouless has been a topic of discussion several times in the past, and now this state is here to stay. A lot of utility, be it spiking, spinning, healing teammates, etc. is provided mainly by bulky waters, and the absence of Raikou makes stacking waters on the same team easier than before. Some of it has been observed already, I feel like I've seen standard defensive Vap more often in Raikou is Kill than in other tours, and part of that can be explained by Raikou not being here to punish a Vap having to stay to get its own Wish as Raikou gets a free turn to set up.
Another notable one is Slowbro, which doesn't have to worry about Raikou to come in and happily start a one-sided Calm Mind war. A side effect of losing Raikou is Regice becoming more likely to give up Psych Up and thus more beatable, albeit still a significant hindrance, for Slowbro. These two impacts might make Slowbro much more competent against offense in particular, and the same goes for Golduck which has been toyed with.
The water vs water matchup might become more common as a result, to the point that I could see several builds relying mainly on their own water to handle the opponents'. If such a trait happens, it could be in favor of some like Lanturn of course, but also Lapras, which has several tools that play in its favor to win these kind of dittos.
Raikou was the best Electric-type in a tier with plenty of Water-types, one of the best wincons, lead, and UUBL premier form of speed control. Among alternatives to fill that last role, Alakazam is the one with the most similar profile, packing even higher Special Attack and Speed and now enjoying some additional freedom in terms of moveslot and opportunities with Registeel leaving alongside Raikou. However, it is still harder to pilot due to its frailty and the vulnerability to Houndoom which can makes preemptive Psychic clicks risky.
Sceptile is another Pokemon that saw a lot of development in Raikou is Kill. While it suffers from a lower Special Attack and a weak and easy to cover STAB which hampers its ability to revenge kill, its SubEndevour set proved to be a very capable progress maker against a lot of teams, usually being able to force a worthy trade. The threat Grass STAB in Overgrow and Petaya Berry range is also alongside the potential sub or Endevour on a switch in is also quite a weapon to have against the many Water-types.
Similarly, Swellow sub Endevour set on a Spikes immune Pokemon proved to potent, as is Baton Pass and a boosting berry on that same set. Swellow's superior Speed is also very good to have against the aforementioned speedy Pokemon as they are much frailer than Raikou and thus can be more reliably revenge killed by Swellow's STAB. That last benefit also helps Crobat, which also enjoys the fact that the most common Sludge Bomb immunity in Registeel is no more, although it still is easily handled by a lot of other Pokemon.
Another category of Pokemon that saw a good amount of success in Raikou is Kill are the fast Normal-types, now less prone to being revenge killed by Raikou. Dodrio's Quick Attack also becomes more potent as the other forms of speed control are frailer and sometimes willingly put themselves in range by using Substitute over and over, potentially leading to more opportunities to click its STABs. Speaking of which, these clicks might just become easier with Registeel leaving the picture, although aside from Omastar, the other common Normal resistances tend to be even better at this job due to their even superior phsyical bulk (Regirock, Steelix) or lack of Earthquake weakness (Scizor, Armaldo, Dusclops, etc.).
And then there's Miltank, the very strong wincon with tons of utility even when it's walled, which may have an easier time setting up if the meta ends up being slower and bulkier and has the added benefit of not having to fear a potentially losing Curse war against some Registeel variants. A slower meta would also beneficiates Ursaring, candidate for the most self sufficient wallbreaker in the meta.
Another thing I'm unsure of is the future of special offense. We did see CM Pass in Raikou is Kill and some recipients or standalone special threats now don't have to worry about Registeel and maybe Psych Up Regice (as much) anymore. Raikou could be a problem for many of these Pokemon, as even if it could hardly safely switch-in, it could use them as CM bait and was favored to win CM wars due to its natural speed and special bulk. However, that also means special sweepers lost a teammate and (nearly) mandatory member of dedicated special offense. Registeel leaving is on paper good news for a lot of special sweepers, but those might not appreciate having to face a phazing Articuno that breaks CM pass chains or an Umbreon that stonewalls most Psychic-types more often.
Might be a case by case, but Ground-types as a categeory might lose from those shifts, taking away the two most important Pokemon they were used for in the first place. Marowak is an exception is the sense that it gained a lot of attention in Raikou is Kill tournament that introduced a slower, bulkier metagame that made its wallbreaking potential all the more appealing, but Registeel leaving means a common entry point is also gone. This applies to most Ground-types, and just like Steelix, the more niche ones that were mainly used for their Raikou matchup might just lose all relevance, especially with Registeel leaving as well and non Earthquake weak bulky waters like Vaporeon, Slowbro, and Blastoise becoming even more prominent.
There's a couple of more Pokemon that I thought of talking about (Regirock, Cradily even, etc.) but this post is long enough and I want to end it. Expect some changes in the sample teams list as most are now obsolete in the future, before or after ADVLTL.
Thank you for reading, even if you didn't.