Even if it may be a bit late, I'm gonna throw my thoughts in too. I don't have much time, so this post won't be as long or detailed as I would like it to be, but something is still better than nothing at all. Furthermore, if I mention something that has already been mentioned, I am very sorry, but I don't have the time to read all the posts atm.
In my opinion the problem with the -Ate abilities is much more complicated than most people think it is. There -Ate abilites have many traits that make them really good, but, atleast in my opinion, it is just many of these aspects combined that push them over the top. I don't think there is the need to ban all of the -ates right of the bat (as others have mentioned, Levitate most broken thing ever imao). The problem I can see with all the options in the poll that they only remove one or two of the positive traits that all the -Ates have to offer. Even if these traits are removed, I can see the positive traits left making the Ate-abilities still broken.
I'll probably miss one or two, but here is what I think what the most important of these traits are:
1. Powerful (ab)users: Mega-Mewtwo-X, Mega-Rayquaza, Mega-Diancie, etc. This has been covered already.
2. -Ates giving stab and a free LO boost (additionally giving x1.3 is insane). This has been covered already too. Banning STAB-ate would help a bit, but not fix the general problem imo. Moving on.
3. Extremely good moves to use the -ates with: That's something very obvious, but it's still important. Moves like Fake out, Extreme speed and Boomburst are one of the best offensive moves in the game, and the Ate-users utilise them to their fullest potential.
4. Only a few checks and even less "counters" (that in most cases don't do anything else besides being a pivot and check/"counter" the ate abusers): This also has been mentioned (sorta). There isn't much that can take a Fake out + Espeed + Boomburst/Techno blast combo once, not to mention more than once. What also reduces the amount of checks and counters drastically is the fact that the most common Ate-users are mixed, thus many potential counters are even less reliable (for example steel types such as Mega-Steelix and Mega-Aggron).
5. Easily being able to wear down the checks and "counters": If you switch in your "counter", which is probably one of the Regis, you can't just "roost off" the damage, moves like Boomburst just do way to much, neither you can PP stall the Ate-user out (in most cases), since they almost ever have atleast the same amount of pp than you. The incredible power of the -Ates basically forces people to run regenerator on their checks/"counters". Which in most cases makes them only usable as a pivot and Ate-user-"counter" (what a word lol).
6. "Oh, you have a "counter"? Oh look, what coincidence, I have a coverage move.": The main reason why I wrote "counter" everytime, rather than just counter, is that every -Ate user can just easily run coverage that destroys/2hko's nearly all of the counters. Thousand Arrows/Precipe Blades/Magma Storm became very popular moves on all -ate users. If you don't have the right mon that can switch into all of the -Ate users moves (in most cases there is one, since they have to check opposing imposters), you're screwed.
7. Extreme utility vs. Imposters: That's a trait that I feel like often gets overlooked. There are many dangerous strategies in the BH metagame, but most of them, especially setup sweepers, are just less viable because you have to fear to get swept by an Imposter (for example the infamous Protean Mega-Mewtwo-Y). That however, isn't the case for the ate abilities. On the contrary to all the other relatively broken strategies checked by Imposter, it's still very easy to switch into an impostered -ate user, atleast if you have prepared "counters" (you obviously shouldn't run coverage that can destroy your check or "counter" lol). Not to mention imposters cannot switch in without taking tons of damage. And since most of the -Ate-users don't run recovery, it's easy to wear imposters down if they have to switch into an -Ate-user.
In my opinion it won't have a big, if any, effect on the current meta if we restrict the -Ates to only one user per team. -Ate spam isn't that common anymore and one -Ate-user alone has the power too completely murder a whole team, even if it's a prepared one. You might aswell wait until lategame and then clean up with your one ate user. As I've previously mentioned, I don't think it's necessary to ban all the ates right away (that would make setup much more threatening too, but that's another topic). In my opinion the best solution would be to combine bans like only one -Ate user per team and no STAB-ate. That would probably the most complex ban ever though, so I don't think something among these lines would be practicable (sadly).
Anyways, thanks for taking the time to read my walls of text, my post became much longer than I've intended it to be. I'll probably add calcs and examples when I have the time, but that's it for now.
-TTer
P.S.: I apologize for all the mistakes I've made, my English isn't all that good. If you find any mistakes, feel free to point them out (be civil though).
Edit: Corrected a few mistakes I've found. Still not sure what to vote... :/
Edit II: I think I would probably vote on restricting the -Ate user to only one per team and see if it makes the -Ates any less broken. If not, No STAB-Ate sounds like something that should be considered too.
Edit III: Corrected a few grammatical mistakes. Didn't change the content though.