Easter eggs in movies are, generally speaking, confined to comic-book movies.
After all, they come from a source material that's far too in-depth and rich with in-jokes for it not to be a thing, so how directors often pay homage is by inserting little nods and winks along the way.
There's countless examples of it in early movies of the Marvel Cinematic Universe, and even something like 'The Dark Knight' featured a nod of sorts to Michael Mann's 'Heat'. However, for Todd Phillips' take on 'Joker', there are none.
Absolutely none. Not only that, any of the ones you find are simply there by reason of you seeing what you want to see. That's not us saying it, either, but director Todd Phillips and Joaquin Phoenix in an interview with Collider.
"I don’t do Easter eggs. Any Easter eggs anybody finds is a mistake," Phillips said, being pretty unequivocal about it. Phoenix, meanwhile, followed up on Phillips' point by making a reference to the infamous Paul McCartney hoax that he was dead.
"I was saying to the last press, they asked me this thing about the clock and 11:11 and I said, “Paul is dead.” And he was like, “What?” I said, “Paul is dead.” He’s like, “I don’t know.” And the other guy that was like my age was like, “It’s a Beatles reference, dude.” It’s these fucking 22-year-old kids, they don’t know “Paul is dead.” I go, “You see what you want to see, you hear what you want to hear.” Everybody thought it said “Paul is dead” backwards and shit."
The 11:11 thing was apparently a theory doing the rounds that the movie was set inside Bizarro World, which was a comic-book storyline about a world where everything was the exact opposite of the real world.
So, there you have it. No Easter eggs in 'Joker'. That's it. Show's over. Go home. Move towards the exits.
