Not OP, but such a card is an insult to standard ccg design. The ability to completely disable a defensive mechanic for 4 mana is ridiculous. CCG’s are all about an arms race between defensive decks putting in more defensive mechanics, such as taunt and heals and aggro decks putting in more reach, burn, and finishers.
Kayn is reach, burn, and a finisher in a neat little package and completely nullifies any taunt minion you play. So stabilizing in the end game is impossible. You have to win the early game hard just to prevent kayn from maximizing damage. Kayn really should only apply to himself in the most fair sense. I don’t even care if he had leeroy’s old attack.