Kassawin

Partially, they removed it since it didn't really add a counter play mechanic since he could silence you, get in and out before it ended. But the blinks are what tied his kit together and made it all an abhorred mess,

That's not even how mobility works.

His mobility has been nerfed indirectly (although you are correct, they dropped some nerfs directly on Kass as well). Being hyper-mobile in a game populated by immobile champions is an enormous advantage, where being hyper-mobile in a game where the majority of meta champs are mobile (some even rivaling his mobility outright) is only a moderate advantage. Since season 3-4 when he was perma-banned and received his rework and kit tweaks, Several new champs with comparable mobility have been added, several older ones have been reworked to have more mobility, and items have also been introduced that further amplify mobility creep.

Simply put, Kassadin's mobility is no longer game breaking, due in part to the rest of the game catching up, and in part to him being toned back.

Which him? Zed or Kass?

Ahh, but by the time he gets relevant (assuming he didn't get fed) Teamfights have rolled around and unless you guys get scattered and all he needs to do is cleanup, you should be A-Ok.

But that doesn't happen in most scenarios. Kassadin is equipped with tools to GET fed, Roams like a god (hence the importance of MiA's) if he isn't already fed midlane. That's why he is such a monstrous assassin. He can GET fed, AND he's a late game hyper assassin. Now, to be fair, if you know what you're doing, you wont let him get fed so easily, and he'll scale at the same pace as the rest of you, albeit with some power spikes at 6 and a fully stacked / And again, don't get scattered, peel properly, and he's a cakewalk, not a riftwalk.

Jeez, do you even rift bro?