So, now onto the next question: how is this going to affect other Pokemon?
Hydreigon will almost certainly stop running specially defensive variants now that it's no longer needed to check Spectrier. It's just a worse Latias for the most part as a phat Dragon-type Defogger, though in terms of basic utility range the two aren't actually that different. Something I hated about using Hydreigon as my Ghost-type resist in the Spect metagame was that it just opened up a horrible weakness to Dragapult, so having a bit more breathing room to run alternate Ghost-type responses on offence will probably prove extremely valuable.
The biggest advantage Latias has as a bulky Dragon-type defogger is the higher base Speed, notably letting her force out offensive SR Garchomp, and noticeably better natural bulk (92/90/90 --> 80/90/130), but beyond her range of coverage is more diverse than Hydreigon, notably having access to BoltBeam, STAB Psychic/Psyshock, and utility attacks like Mystical Fire (as well as the far less noteworthy Mist Ball). Utility-wise, Hydreigon's range wasn't
that bad, having access to good moves like Thunder Wave, Tailwind, U-turn, and Taunt, but it suffered from being kinda forced into running both Dark Pulse and Earth Power to do its job properly. Latias doesn't really suffer from that issue due to her better-specialised stat spread (making a lack of direct attacks much less of a problem), and her utility options are better suited to a general utility role than Hydreigon's, notably having access to Trick, Future Sight, and the extremely powerful Healing Wish over Hydreigon while sharing access to Thunder Wave and Tailwind for indirect speed control.
Offensively, Hydreigon faces heavy competition from Latios and Gengar, the former of which is just a more potent special-attacking Dragon-type between its access to Trick, better STAB combo, and higher Speed/SpA stats and the latter of which is a faster Nasty Plot breaker that doesn't need to play games around Fairy-types and Blissey. Hydreigon does have a few things going for it over Latios, namely access to Taunt, higher attack+Superpower, and more firepower on certain key coverage types (Mystical Fire --> Flamethrower/Fire Blast, Aura Sphere-->Focus Blast, 90 Atk EQ-->105 Atk EQ+Earth Power, etc.), and Nasty Plot>Calm Mind, but I would be very surprised if that's enough to salvage it when the metagame around it necessitates such awkward coverage for Hydreigon (it's one of the few 'mons in this meta that I think has REAL 4MSS—not the fake 4MSS that everyone seems to always use the term for, I mean it literally needs 7 moves if it wants to do its job properly). All in all, Hydreigon looks like it is going to struggle to carve a niche in this metagame, especially given that it can't rely on Dugtrio to trap blobs like it did towards the end of gen 6.
Gonna be a bit more brief on these two 'cuz there's a lot less to unpack. In the Spectrier metagame, they were kinda pigoenholed into running suboptimal sets to not just lose to certain Substitute Spectrier sets. Now Mandibuzz is free to run a single Dark-type STAB/forgo Whirlwind in favour of something actually useful and can also afford to run a more useful EV spread. Blissey is less drastically changed than Mandibuzz, but no longer needing to run Shadow Ball relieves a lot of strain on its range of utility—Thunder Wave, Healing Wish, Stealth Rock, Toxic, Aromatherapy, etc. can now all be run alongside the combination of Seismic Toss and Teleport without needing to worry about Taunt Spectrier steamrolling you. All in all, beneficial changes. Whether Mandibuzz will get more or less popular is another matter, but I can't imagine Blissey going away anytime soon.
I mentioned these when talking about Hydreigon, but they also directly benefit from the absence of Spectrier, so I'm gonna bring them up again. Latios no longer being forced out by a giant horse-shaped win button is a big improvement over what it was before. With that said, it still struggles with Magearna and Cinderace, so until those two get banned I can't imagine it being
too insane. Sinking momentum versus broken Pokemon is still pretty much a death sentence for a Pokemon, even if they would otherwise probably border on being broken (just look at poor old Keldeo). Gengar is in much the same boat as Latios, albeit with a better Magearna matchup, so I won't repeat myself too much. Both of these Pokemon also
dispise Dragapult, who is looking primed to retake the number-one spot in this format now that its main hyperspeed Ghost-type competition is gone. (More on that below.)
Dragapult is a menace. I think there are probably legitimate arguments to say that it is a broken Pokemon (though not banworthy), given its insane Speed stat, aptitude at spreading status, how tricky it is to switch on in the late-game stages, and its near-universal utility for teams. But unlike a lot of the other things that can be described as "broken," what Dragapult brings to the format is ultimately very healthy. It's a lot like (more-so past-gen) Landorus-T in that regard: very easy to plug onto teams, arguably "broken" but in a healthy capacity. Of course, Dragapult only gets better with Spectrier gone. Sure, it loses one of its niches as the Spect Killer, but in exchange, +Atk/+SpA natures are back on the table and you no longer need to justify bringing it over Spectrier. I genuinely think Dragapult may be the best Pokémon in the format right now, and I'm excited to see how its sets change with Spect gone (expecting a mix of IoA and late-stage DLC0).
Tyranitar will probably still run specially defensive variants, but they will no longer be forced to carry Rest to not just crumble to Spect, and CB sets are now back on the table to act as a breaker on sandstorm teams once again. I think it is overall probably in a worse place than in the Spect meta, as its role will become less valuable, but it still has a lot of good things going for it, such as checking Heatran and Dragapult.
I know, this is a weird 'mon to bring up given it's not incredible, but it's probably better now that it's not free real estate for Spectrier, so I thought I'd mention it anyway. Mew is very versatile, and especially defensively it has a lot of things going for it: access to Spikes and Stealth Rock make it a versatile hazard layer; access to Defog makes it a decent hazard clearer. access to Teleport, Will-O-Wisp, Transform, Knock Off, and reliable recovery makes it an even more interesting utility choice; and perfect offensive coverage means it can pick and choose exactly which hazard setters/clearers it has a good/bad matchup versus according to the needs of its team. I think Mew's pretty unexplored, so I hope people will give it a whirl now that teambuilding has been freed up.
There is so much else I could probably talk about, but I've already wasted enough time on this thread and CBA to think about it more, so I'll stop there. Interested to see what other changes people expect.