In my opinion as a long time Twitch player: he doesn't need nerfs. Currently I believe he is in 1 of 3 states:
**-He is balanced.
-Everything else is too weak.
-He needs a rework.**
Nerfing damage won't help unless you go past line of "he's playable". He is a hyper carry, so he will _always_ do lots of damage mid-late.
Nerfing anything about his stealth won't help either because he'll still be able to do what he wants to do: assassinate lone squishies and teams. He may have to slightly adjust how he approaches it, but due to how stealth functions it will _still be happen_.
With this in mind, **either everything else is too weak** or **he needs to be reworked**. In the rework case I would suggest he either needs his stealth removed and aoe buffed to compensate (not my choice of rework considering how stealth is heavily a part of his character identity) or aoe removed and stealth playstyle buffed.
This is what Riot needs to work out imo: is he balanced, is everything else too weak or does Twitch need between a Rek'sai and Warwick sized rework.