It's after the series ended, with that being- some things have happened. two things are that All For One is dead, and All Might is no longer Number One
League of Villains no more, despite the members being alive- and not much of a threat due to not meeting One For All

How did One For All die exactly? and what's different?

first off, League of Villains still is ran by Shigiraki. and something different happened in the finale of the fight against UA and One for All-
-It went through similar to how it actually started, only thing different was Izuku wasn't separated from Bakugou and the others. so, he successfully defeated One For All, which made the rest of the ending irrelevant- making all the fusses from the series that happened in the final showdown different

But a few years later, TWO villain groups started rising and becoming much worse than that final fight against UA and All for One. those two groups being Zodiacs and Seven Deadly Sins
and one of the many new hero groups are called Seven Heavenly Virtues

Will the Heroes or Villains win, who knows...