Stealth is easily the most toxic mechanic in League, and has no direct counterplay. If someone stealths and is able to get right behind you in lane, you just dead. You had no idea they were 5 yards away from you until they dump their entire kit into you and kill you.
remove it from the game entirely, replace stealth abilities and if necessary rework the champions its on. Or, at the very least, massively increase the cooldown on all stealth abilities. Seeing a twitch engage, kill your team mate, then immediately restealth before you can catch him is just ridiculous.
I expect to get downvoted to oblivion but whatever.