Regarding Zacian, I think there’s been a fair amount of pushback against the ban idea because some people don’t really see it as broken in practice.
I'd like to start right off the bat with this: Zacian has viable counterplay. However, I don't think this means we shouldn't ban it.
In truth, Zacian is really only 100% manageable with Necrozma-Dusk Mane and a Regenerator core - the main ones that stick out to me are Tangrowth and Toxapex. With Ho-Oh, it's a lot better than just having NDM, but you are really cutting it close when any rogue Wild Charge can one shot you. Tang and Pex are a lot more reliable in this matter - being able to take pretty much anything Zacian can throw at it, even when boosted. You'd need specific circumstances like rocks up to beat Pex or tspikes up to beat Tang. In theory you could go freeballing and run just an Amoonguss to beat Zacian and call it a day, but you are really cutting it close should Zacian sets adapt to the meta and start running Psy Fangs. But okay, sure, this is certainly viable counterplay - be it extremely specific counterplay. You could say that means it isn't broken, but it certainly is pushing the boundries at the very least.
Now, we also have to establish that not all players loves running Tang or Pex - and you also have to consider the other mons that are pretty much forced onto every non-HO team. Basically, you are looking at 4 mons which have to be run on almost every team - Yveltal, Etern, Nec, and a regenerator mon. So, alright, fine, there are some small variations, such as Blissey over Etern, but this also usually comes at a significant teambuilding cost, such as forcing a Poison-type or dual defog on your team, lest you lose to Toxic Spikes. But these 4 basic slots are pretty much it - and this is severely limiting to teambuilding. If you want to be sure that you won't lose to HO, then you'll likely want a scarf or priority revenge killer. Want to not lose to balance or stall? Alright, well now we need a cleric mon, too. There are so few options to be able to build from this core of 4 mons that it hurts to think about.
This has been so frustrating to the playerbase to the point that people are now running egregiously unoptimal builds in an attempt to be original and actually try to have some fun in the tier. The number one example of this I've seen so far this UPL is Necrozma-DM as the only switch-in to Zacian. Guys, I don't know where you thought running one mon that almost always gets 2HKO'd would be in any way a reliable method of dealing with Zacian, but it's not. Also, I find points about offensive counterplay to Zacian to be grasping at straws, at the absolute best. Timid Eternatus is literally so weak, and it will likely be your best Knock absorber considering nothing in the tier can really take knocks like etern can, making it even weaker. Marshadow is also extremely weak if it goes Jolly, missing key 2HKO targets on switch-ins such as Eternatus. If Marsh does stay Adamant, then it has to use Shadow Sneak - which does 44% max to an uninvested Zacian. This is all not even considering that Jolly Zacian is totally viable to run on Babiri Berry sets. Even Calyrex-S will not OHKO Zacian unless it's specs, which is still a roll. Thus, if you want any reliable method of dealing with Zacian at all, you are forced into a.) running the same 4 mons on every team or B.) going full HO. There really is not any inbetween, which is why, as I've said before, Bulky Offense is basically an impossible archetype at the moment.
While there is viable counterplay to Zacian, we should not forget that Zacian is the number one offensive threat in the tier right now. I think a flaw in the logic of the No Ban supporters is that they look at the mon in a vacuum - Zacian can also rely on its team for support too. Sure, if you just load up icemaster’s old sample team and expect Zacian to break everything in the meta right now by itself, you are going to be out of luck. But Zacian itself pairs excellently with both entry hazards and other offensive threats. The classic double fairy core of Zaci + Xerneas will still absolutely murk the majority of teams (and it has in UPL) since NDM easily gets overloaded and additional Xerneas checks are dwindling when Regenerator Mons are forced on every team and there’s no more room for checks to other offensive mons. Babiri Berry Zacian makes an excellent cleaner and breaker, and pairs excellently with Healing Wish Blissey, as most teams can’t handle a Zacian which has broken through NDM and gotten back to full health. This is not to mention the surprise factor of the SD set which can take advantage of predicted switches for a CB set. Even just having rocks up (not the most difficult thing in the world, especially when one of the best defoggers, Yveltal, doesn’t like Zacian) will allow CB Zacian to wreck havoc and break through some of the lesser-used regenerator mons such as Amoonguss and Toxapex. By the way, quickly going back to the argument of offensive checks: Zacian is one of the fastest mons in the tier, so it can massively push it’s advantage, much more than other breaker mons. In comparison, mons like Don or Zekrom, while having massive breaking potential, need to set-up more than Zacian does to break, otherwise random things like Yveltal and Etern can come in, outspeed and get enough damage in to limit their presence.
As a last note, the idea that Zac will prevent Stall from taking over wholesale is just baffling to me. Imagine how offensive your team can be now, when you don’t automatically need a passive NDM and Tang/Ho-Oh! This is also not even considering how Zacian's power as an offense mon automatically makes you want to use it over something else. Getting rid of a fast sweeper like Zac allows a lot of the slower, but still very strong breakers to rise up. Another thing that is baffling to me on this topic is that the same people who would argue about stall taking over would, at the same time, argue that Zacian is very manageable to deal with. If Zacian isn't an extremely strong offensive force, then wouldn't it not matter if its around or not for stall? Anyway, I don’t want to speculate past this because, as fardin excellently pointed out, there’s no point in speculating in a hypothetical metagame. It’s more important to deal with the current state of the meta, because we already know it’s very limited for creativity and teambuilding.
The mon is simply not fun and is massively limiting for the tier. Due to this I think it should be banned. You could certainly try to pull the "well, this is Ubers so...." argument, but let's not pretend Ubers has to stay on the old ideology, especially when we've already banned Arceus, Mega Ray, Zacian-C, Shadow Tag, etcetera.... a mon of comparable power-level in any other tier would for sure be banned. It's important to be ideologically consistent - saying Zacian should not be banned because it's not quiiitteeee as broken as Zacian-C is not something that would be done in a "real" tiered meta like OU, nor would it be done in the wild west of Uber's past. If you are going to have those principles, at least stick by them like the ultra boomers do. Though, at this point, I just see that as "everyone else can't enjoy a fun balanced meta because I like broken things."
