It’s true that greninja has been a part in the ranked pvp for almost 2 years but back in that time we still don’t have any pvp council yet. The council was created to discuss about any pvp topics because the staff might be not knowledgeable enough or need some pvpers’s opinion to discuss about pvp things and I believed the thread about greninja and aegislash were brought up to discuss since the council was created but they did not get banned for some reasons even if the council members agreed to ban it for long long time.
Why they unbanned metagross ?
Some people in council agree to ban it and some people did not therefore the council do their job to survey top 25 pvpers’s opinion in both server [in that time] last season which is in April, 2021. I guess, the conclusion of the survey [ both of council members and Top#25 on both servers] is unbanned the metagrossite. below is my opinion about metagross.
We are going to talk about “How greninja broke healthy metagame” In my viewpoint.
There are lots of thing that can check greninja such as chansey, ferrothorn, rotom etc. but none of them can counter it.
Chansey : low kick is not even 2 hits k.o. but there is the chance to do that with stealth rock up and high roll [in case of full hp] and free to set up spike predicted chansey is to switch in as a checker then greninja can safely switch out. [stay in and get toxic, paralyzed never be a play to be honest]. Spike is the best support move for greninja to help any sweepers do their job easier. You know the move Defog and it’s exist in PRO ?
Almost of the defoggers in PRO meta are weakness to greninja :
 Rotom-wash : Hp. Grass, spike [predicted the volt switch]
 Ice weakness defoggers : Zapdos, Tornadus, Skamory and gliscor etc… [ hydro or surf]
They are all slower and weak to icebeam, surf therefore they can’t defog in front of greninja.
Ferrothron : absolutely fine to check greninja with out hp.fire and low kick but as I said above free spike on predicted switch and don’t switch out and take leech or some damage are not the play.
Clefable : Gunk shot doesn’t 1 hit k.o but it doesn’t good enough to check, tbh
Conkeldurr : sneak with protean or risk to get k.o. with match punch when staying in water-dark type is not a good play.
Greninja is fragile as you said but if you can use it properly you won’t let it take any damage, of course it’s fragile compared to metagross but there’re things can check and counter matagross imo. it’s not that hard to deal with [ sucker punch, scarfted magnezone forced to take damage from volt switch, some safe checks. Etc.]
Grenninja’s movepool : of course every pokemon has 4 slot of moves but with its ability, I would say it’s too op because it’s stab every move and before you figure it out what set it is, your team might get ruined, of course we can check the move by double switching to check but if the hazards are up, I’m not preferred to take additional damage for free and it can learn u-turn, get a free momentum yeah ….. boom!
It can be dealt with all major playstyles with only 4 moves : due to the flexible playstyle and movepool I think it is because as I said we can’t figure out what set it it.
All I said above about the greninja, I’d said it causes the unhealthy meta. I considered those circumstances above as 6vs6 not 1vs1 because when you pvp, you have 6 pokemons as your opinion as well and to clarify this only my viewpoint why greninja should be ban.
All of discussion banning pokemon I think the council made their decision with their respectable knowledge, some surveys with pvpers and they carefully consider about it, I believed. I understand you point, all of PRO players are not pvpers how pvpers can be representatives of all the people in PRO community. You might feel like your voice doesn’t be heard but It doesn’t mean they did not care about your voice. Some duty might need specific skills or people who are knowledgeable knowing pokemon in competitive way enough to get the best result of any discussion. They just “Put the right man in to the right job”.
Additional : https://www.smogon.com/smog/issue44/ou-to-ubers
That’s why I “-1” this thread.