I have a hunch Kayne's shadow move was a test for Eve's upcoming update.

If you read Kayn's development, the wall-walking ability was actually there since the times of Ao Shin, where a certain rioter planned an ability that got a nickname "true flight" and soon became a meme among rioters to the point that whenever there was a champion in a development, they instantly shouted "True flight!" as one of ability concepts. We can see the Coment of the Legend in 's kit as one iteration of the true flight, not that flexible though as Kayn's. Kayn got to be THE champion that could use the true flight in his kit without it being broken (in the way it works). I'd even say Talon's parkour is another attempt to utilize a form of the true flight.

Yeah Riot has a habbit of doing stuff like this.

For example when updating Zyra plant A.I they released Kindred shortly afterwards with wolf using the same mechanics.

i guess shadow assassins hair are a hunch for new eve's hair.

Was there not a second leak somewhere along the line which said something more about Evelynn's in-test stealth mechanics, including a wall-walking ability?