Justice League, but the poor critical reception of those films has led to Warner Bros. changing approach.
Now, while most Joker was set.
However, Suicide Squad (of sorts, at least). It builds on certain character and story arcs from that movie, but at the same time it puts being a standalone first and foremost, which means there are some DCEU plot holes too.
Gotham City Is Missing A Lot Of Key People
The action of Birds of Prey takes place in Gotham City, which makes sense since it's where the titular characters - including Harley Quinn - typically tend to operate. However, what doesn't quite add up is how there's so much activity in Gotham that doesn't attract the attention of any of its more notable denizens. As already established by the DCEU - across multiple movies, no less - Gotham is home to a very much active Batman, a crime lord take on Joker, and a well-established Commissioner Gordon, none of whom appear in Birds of Prey (save for flashbacks to The Joker).
Looking at this from an outside-the-universe standpoint, it makes sense: Jeffrey Wright is replacing J.K. Simmons as Commissioner Gordon, so logically neither of them can turn up. There's no in-universe explanation given though, despite references to Bruce Wayne and the amount of high-profile illegal activity going on in Gotham City. Harley Quinn on the loose and going up against Black Mask is the kind of thing that would ordinarily capture the attention of both Batman and Gordon, but neither appear here. There may be possible reasons for this - Batman could be off investigating something elsewhere, Gordon could have his hands full with another case - but none is offered up by Birds of Prey.
The same goes for The Joker, who is a key driving force behind the Harley Quinn movie because of their break-up. But while he is acknowledged and appears in flashback, the Clown Prince of Crime doesn't appear in Birds of Prey's central story. Again, there might be explanations, but given how he's spoken of as the big cheese in Gotham City, then it's still a little jarring for him to be so absent in-universe.
Amanda Waller & ARGUS Don't Try To Get Harley Quinn Back
At the beginning of Suicide Squad, Harley Quinn is imprisoned at Belle Reve Special Security Barracks, alongside the rest of her future Task Force X teammates. While she's let out as part of squad, she and the rest of the Task Force go back to prison after the mission is completed, before The Joker breaks Harley out at the end of the movie. Fast-forward to Birds of Prey, and Harley Quinn's movie finds her not only free from Joker, but free from prison as well.
That's all well and good for Birds of Prey, since its plot necessitates that Harley be around in Gotham City. But it also makes a DCEU plot hole, or at least a mystery, for why Amanda Waller and ARGUS haven't re-captured Harley Quinn yet. She was considered a highly dangerous criminal, and it's not like she's on the run - she's quite openly moving around Gotham in Birds of Prey, and so theoretically should be an easy target for Waller. Of course, things didn't go too well for Waller in Suicide Squad, so that could have something to do with it, or it may be that this is necessary setup for Harley Quinn's role in James Gunn's Suicide Squad 2.
Captain Boomerang Is Wanted In Gotham (Despite Not Being His City)
Like all comic book movies, wanted poster for Captain Boomerang, who was played by Jai Courtney in Suicide Squad. Harley Quinn pauses when she sees the poster, exclaiming "Hey, I know that guy!" The poster confirms that Captain Boomerang escaped from Belle Reve sometime after the events of Suicide Squad, which isn't too much of a surprise, but the fact he's wanted in Gotham is.
As established in the DCEU, Captain Boomerang's usual territory is Central City, hence Ezra Miller's Flash apprehending him in Suicide Squad. It's unclear why, then, Boomerang is wanted in Gotham, which suggests he's recently been active - and may still be active - in the city. This also makes the absence of Amanda Waller and ARGUS even stranger, since it means at least two of her Task Force X are out of prison during the events of Birds of Prey.
Birds of Prey's Joker Doesn't Look Like Jared Leto
The Joker may not appear in the present day events of Birds of Prey, but he does still make some appearances in the Harley Quinn movie. They too, however, create a DCEU plot hole of their own, because the Joker we see in Birds of Prey isn't played by Jared Leto, and for the most part doesn't even look like the character introduced in Suicide Squad. In Birds of Prey's animated sequence, while Harley Quinn looks the same, the Joker appears much more in line with the version from Batman: The Animated Series. It's a traditional take on the character, and nothing at all like Leto's controversial appearance.
This continues later when a drawing of the Joker is shown in Harley Quinn's apartment, and again doesn't look like Leto's Joker. Even when the character appears proper in Birds of Prey's flashbacks, we don't get to see his face. Again, on a logistical level that avoids having to bring Leto back, but more so it also sets up a Joker recast for the DCEU, and one who will likely be a rather different take on the character than Leto's, despite ostensibly existing in the same universe.
