Is Yasuo still OP?

I think he's been successfully booted out of high-tier, yeah. There's still the crazy synergy he has with Malphite or Wukong, but he probably cries himself to sleep if his lane opponent has the slightest bit of poke that's actually constant.

Yasuo would probably be considered under-powered now if anything.

Although Riot left (mostly) his offensive capabilities intact, Yasuo does not have the defensive stats commonly found in fighters to back it up. He will loose a match up against any other melee carries with equal gold (Yi, Fiora, Tryndamere). Additionally, he can't survive a full AP cast rotation of most mages and AP assassins.

To play Yasuo, one has to maximize the potential of sweeping blade and wind wall to the fullest; he cannot afford to be hit by the slightest of anything now days.

I think he's rather underpowered. But he's still perma locked in low ELO. He's just far to weak early and mid game, and doesn't have enough health for late game.

Though he is very entertaining to play, he is usually too weak to be a viable champion, and mostly everyone fails with him. If you want some hard evidence watch the high ELO players play him. About 99% fail with him.