With a 43% win ratio (and dropping) 
needs some serious help. His mobility needed to be nerfed but his damage was nerfed repeatedly instead. His ult scales with mana? Why? His farming is ok but for the most part you will never get to farm since you just die and do no damage back.
With a lot of other assassin offering similar mobility and 2x more damage I just do not see kass being use full anymore.
Good riddance, now they should olaf fizz and the game is much closer to real balance
Win ratios does not mean a champion is underpowered.
His mobility is pretty much the same?
His farming is great for under the tower, W gives you bonus damage.
Kassadin is one of the best mid laners to follow up with a jungler ganking.
The reason he loses alot of games, is just because you want to play a certain champ, doesn't mean your team comp will support him.
What does kassadin lack in teamfights? Poke and a big Ultimate. Supports that work with kass have to be AP mages pretty much.
Lux sup --> into late game BIG ult, nice teamfight control Zyra
What does not work with kass? Single target champions.
Fix it with an AOE team comp. Miss fortune, Malphite, Kennen, Fiddles
NOT
Janna, Garen, Lee sin, Ryze
MAYBE
Rumble, jarvan, vlad,
So the general idea is -> Shut down lanes. Jungler ganks your lane, kill the mid, clear wave, gank another lane repeat. Your team should be the first ones to 11 or 16
Team fights --> You walk towards enemy, E Q, R away If they commit to you
Your teammates ult.
Simple as that
It's fine that Kass is in this state now. As of now he is no longer a perma ban/pick because his power has been hit hard enough that he can't just easily carry any game he is picked in. To use him now he actually needs someone who knows what they're doing with him and to scale a lot later into the game so he can be useful. His abilities still allow him to farm really well and he actually has some unique build paths compared to other late game AP carries. He can actually go tanky-bruiser if he gets icebourne/FH so he has things going for him that other AP mids/top don't.
Most void style mids are no longer just op safe picks anymore. They have been brought to the point of strong counter picks that need to be used carefully. It is not that kass has been nurfed to hard, It is just people are not used to having to play safer. Most of the time before this people would become majorly over zealous with void mids and be able to make reckless moves and still be rewarded for them. Now they have jumped in terms of skill level and pick order restraints. Kass is still a strong and viable pick again SOME but not ALL.
