Concerning balance and winrates

I'm not here to complain about Yasuo's kit or Riven. I just have some legitimate questions concerning balance and winrates, or rather how they influence one another.

I'll take as an example. I play him quite alot, I have other 200k Mastery points on him. And to be quite frank, I enjoy his current state alot! Despite his Winrates in Plat+ being amongst the lowest in all 132 champions, even in elite elo, he still feels like a very relevant pick. Personally, I pick him into immobile juggernauts like or .

I know Rito looks at winrates, pickrates and banrates, and I believe they take a look into pro play aswell to determine champions states.

My real question is: When does Riot start watching a champion's performance, considering they aim for 50%ish winrate?

Is there an accepted deviation of 5%? Say a champion is considered balanced if he stands between 45 and 55% winrate in Plat+?

sits at approximately 46-47% winrate in all high - elite elos. Yet, his pickrate in pro play is remarkably high. Is he considered balanced? I think so. But does Riot think so? I don't know.

Of course, as I mentioned, is but an example here, there are plenty of champions whom have sub 50% winrates. I am just curious as to how Riot selects champions to be reviewed.