There are only two Assassins that are above a 50% winrate
which are Ekko and Diana. The rest are below and some even far below a 50% winrate. Meanwhile, mages have the top 13 spots aside from Corki. The Assassin update was over a month and a half ago. No other update had the majority of the class it updated under a 50% winrate after that long. It doesn't even look like there are any plans for Assassin buffs so mages are probably just going to dominate mid lane for another season straight.