2016's Suicide Squad may have introduced Task Force X to the DCEU, but the movie also gave Batman villain The Joker a strong supporting role. That's because the focus was very much on the Clown Prince of Crime's dynamic with Harley Quinn, and after Warner Bros. ordered reshoots that completely changed his role in the final act, he was on hand to break her out of Belle Reve.
In David Ayer's original cut of the movie, The Joker would have turned on Harley due to her allegiance with Task Force X, eventually returning during the final battle where she was set to reject him.
Birds of Prey ended Harley's relationship with the villain, so it's no great surprise that he won't factor into James Gunn's The Suicide Squad. During a recent interview with The New York Times, the filmmaker was asked if he ever considered bringing the Harlequin of Hate back and explained why it was important to him to recreate this villainous team in his own image.
"I just don’t know why Joker would be in the Suicide Squad," Gunn admitted. "He wouldn’t be helpful in that type of war situation. I wanted to create what I thought of as the Suicide Squad. For me to react to David’s movie would make it the shadow of David’s movie. I wanted it to be its own thing completely."
"When Warner Bros. said they wanted me to do this, I watched the first movie for the first time, and I called them back and said, what do I have to keep from this movie? And they said, nothing. They said, listen, we would love it if Margot’s in the movie but she doesn’t have to be. You could come up with all new characters or you could keep all the same characters."
Once again, it seems clear that The Suicide Squad is going to fall somewhere between being a reboot and sequel, but you can't really blame Gunn for wanting to move on from Ayer's divisive vision of the team.
As for Leto's Joker, while he recently appeared in Zack Snyder's Justice League, there don't appear to be any future plans to bring the villain back to the DC Extended Universe for the foreseeable future.