Joker: Folie Deux has a few connections to Batman lore, but I’m disappointed that it abandoned one of its coolest changes from the first film. Joker 2 is not a standard comic book film, and follows its predecessor by being a slow-paced crime thriller – but this time while balancing also being a musical. There were hopes that the movie would serve as a follow-up that continued Arthur’s transformation into Joker, but that’s not strictly what happened. Outside a few references to Gotham, like introducing Joker 2‘s Harvey Dent, the sequel is almost completely severed from its DC ties.

https://screenrant.com/joker-2-batman-canon-retconned-coolest-comic-change-disappointed/