Cassiopiea is a Yasuo counter for sure.
My friend had like an 85% win rate with Yasuo and I've never seen him do worse than any KDA equal to 2/1 as Yasuo either. He has mastery 7 as him and nearly every skin. I think it's safe to say he's a Yasuo legend.
He finally lost and got tilted as Yasuo versus a Cassiopeia.
Cassi can cancel his insane dashes with her W, kite him hard with her Q, heal his damage with her Q+E combo, and has a hard cc that lasts a while and isn't hard to land.
Just wanted to put out that Yasuo has a counter I'd like to make known.