and Kha'zix's defensive power was taken out of R, then is Katarina's E (15% reduced damage) next on the list? I don't think she's OP at all, I'm just wondering for the sake of consistency in the balance philosophy here.
Assassins are really close to assassin fighters, where you bump into 100% damage block potential with champions like Yasou, Jax, Master Yi, Tryndamere, Poppy, Wukong, and more.
Nothing enherantly wrong with defensive boosts on a squishy.
Thing with Kha'Zix and Akali is stealth but useful things when you're revealed is kind of awkward.
Also in Kha'Zix's case it tended to cause him to go full bruiser-build with Randuin's and become Alistar status tanky to naviagate a fight, which seemed wrong. Having leap and stealth should be enough assassin-esque survivability.
With Katarina though - Katarina is a melee champion that throws herself into the middle of a team-fight and sits there for ~2 seconds to AoE things. Her mobility isn't about skirting around the battlefield, it's about jumping into a situation she does kind of need durability to work with. Most other AP carries focused on AoE (Mages) can nuke everyone from a pretty safe distance outside of the barrage of team-fight damage, where Katarina sits in the middle of it.