Is Joker part of the DC Extended Universe?
No, Joker is intended as a standalone film and is not part of the DCEU. If anything it would be part of a new universe where various standalone films will be made but even then that sounds like more of an idea for future movies than an idea for a universe.
"It's not really connected to that [DC Movie] Universe," Phillips told the crowd at a Q&A for Joker during the Toronto International Film Festival. "And it was really intentionally not. I mean the original idea when I went to [Warner Bros.] with the idea was not just about one movie, but about a label - sort of a side label to DC, where you can do these kind of character study, low-rent, low-budget movies, where you get a filmmaker to come in and do some deep dive into a character. So it was never meant to connect, so I don't see it connecting to anything in the future. I think this is just this movie, you know?"