yasuo hasn't been op for awhile. he just has a cancer kit to play against for anyone who relies on projectiles and/or is immobile. hes just frustrating.
Yasuo, Lee Sin, Zed, Blitzcrank, Fizz, Camille, Evelyn, Tryndamere are all in this class.
It doesn't matter if they are op, people hate fucking playing against them.
Yasuo has mobility, damage, and shits on projectile/immobile based champions. Lee Sin has damage, mobility, sustain, a strong form of cc, soft cc...just frustrating that its up to the lee player not you to pick your fights. Zed not only shits on immobile mages, but doesn't have a resource so its frustrating going oom since you have to rush armguard. Blitz is the classic "you missed 10 hooks, but landed 1 so now i die instantly" champion. Fizz/Vlad/Lissandra can be maddening with zhonyas as they feel they are almost permanently untargetable in team fights once they get them. Camille will take any immobile champion out of the game if played properly. you literally have to stand over a screen a way and wait for her to blow her shit on someone else or she will constantly dive you and kill you. Evelyn has very little counterplay if she can manage to get even one or two good ganks off. She is balanced by the fact she has hard time doing that, but holy shit an eve who is even slightly ahead is cancer as she will blow you up out of nowhere which limits everything you can do. Tryndamere not dying for 5 seconds means he can dive you if ahead and you need to be able to win without your CC so hard that you can cc him during ult to win after it runs out and even then often he just spins away.
none of those champs besides maybe Lee and Fizz are "strong" right now, but man are they frustrating to play against. this is why they will always be common bans or just hated. tryndamere and eve are the only ones I don't see banned ever on that list.