So Twitch is being ignored by the marksman update despite desperately needing buffs
Vayne's ult gives double the AD his gives, Jinx has a passive 100% dmg splash on her attacks that can crit pretty much all the time (instead of 40% like twitch's)
Kallista's rend can stack "indefinitely" and ALSO has more base damage than twitch's Contaminate. It ALSO slows, and resets on kill.
Twitch's W does no damage, slows, and applies 2 stacks of his passive for pitiful damage. His Q doesn't really have a long lasting stealth until rank 5. Which, by the way, basically never activates under damage (shaco's is a flash, and a stealth and happens immediately regardless of whether or not he is taking damage at the time, and STILL lasts for 5 seconds. 2/3rds of the duration of Twitch's max level stealth, starting from level 1)
TL:DR, Where is twitch's power budget? Against skilled players, his stealth is predictable and not very powerful, and his kit has been toned down in several areas.