Illuminate

Illuminate has undergone a significant change across generations. From Generation III through VIII, it had no battle effect whatsoever — its only function was an overworld effect that doubled the wild Pokemon encounter rate when a Pokemon with Illuminate was in the lead party slot. This made it widely considered one of the worst abilities in the game, as it provided no competitive value and its overworld effect was generally undesirable.

Starting in Generation IX, Illuminate was reworked to finally have an in-battle function: it now prevents the Pokemon's accuracy from being lowered by opposing moves or abilities. This brings it in line with abilities like Keen Eye, giving it actual competitive relevance for the first time. The overworld encounter-boosting effect was also retained. Found on 8 Pokemon including Staryu, Starmie, Volbeat, and Lanturn, Illuminate went from being a near-useless ability to a functional (if modest) defensive tool with the Generation IX update.