Kayn is fine.
He is a late game, skill shot reliant assassin with no escapes and a very short window to do his damage. He requires a bit more map awareness to deal with due to his wall run ability, but that doesn’t make him overpowered at all.
He is one of two balanced assassins right now. The other being kha.
If you’re going to complain about an assassin make it one that’s actually not punishable. Kayn dives in and kills two targets (exactly what he specializes in) then dies because he has no real escapes due to the restrictions on his e and his q being needed in his burst combo.
Zed dives in, kills one person in a split second with all point and clicks and a undodgeable spin, then teleports out. And repeat every 45 seconds.
Kayn is punishable and actually requires people to be out of position to kill
And escape. No other assassin other than Kha Zix has this restriction right now.