>>532492693
Short answer - Season 3
But honestly it was probably still okay (only because the core from the early years was still largely intact) up until around Kaisa era. You could remove every champion released after Kaisa and the game would be better off for it, the only good champion released in the last 5 years is Vex, and the modern game is simultaneously overly complex while being stagnant in feel, because there's just so many systems layered on top of each other.