(Note: This post contains potential spoilers for Peacemaker Season 2.)
David Ayer’s Suicide Squad is canon to James Gunn‘s DCU, as revealed in Peacemaker Season 2, Episode 3, “Another Rick Up My Sleeve.”
In Episode 3, Joel Kinnaman‘s Rick Flag Jr. returns to the DCU with a reimagined backstory: he serves in the army instead of the marines, is dating Emilia Harcourt after her breakup with Peacemaker (John Cena), and is portrayed as more sheepish and clumsy than his original version.

In a flashback set three years ago, it is revealed that Rick Flag Jr. was romantically involved with Emilia Harcourt (Jennifer Holland) before he left and was killed in Corto Maltese during The Suicide Squad (2021).
Enchantress is also mentioned—Rick Flag Jr. says in the flashback that he is still dating June Moon, the sorceress who appeared in Suicide Squad (2016). Presumably, she is still active in the DCU.

Events from Suicide Squad (2016) are canon to the DCU, as June Moon first fell in love with Rick Flag Jr. in the 2016 DCEU film. The mention of their relationship may solidify certain events from that film as canon within the DCU.
In a recent interview, James Gunn, the showrunner of Peacemaker, revealed that the last three episodes of Season 2 contain many surprises.
Gunn said, “I am the one keeping the last three episodes from everybody because there are too many surprises in them. Especially Episodes 6 and 8—they are just crazy. My favorite things I have ever done are Episodes 6 and 8, and I just can’t wait for people to see them.”
In Season 2, Christopher Smith/Peacemaker (Cena) discovers an alternate world where life is everything he wishes it could be. But this discovery also forces him to face his traumatic past and take the future into his own hands.
The cast of Peacemaker Season 2 includes Cena as Christopher Smith/Peacemaker, Freddie Stroma as Adrian Chase/Vigilante, Brooks as Leota Adebayo, Jennifer Holland as Agent Emilia Harcourt, Steve Agee as John Economos, and Frank Grillo as General Rick Flag, Sr. Recurring roles feature Robert Patrick as Auggie Smith/White Dragon, Nhut Le as Rip Jagger/Judomaster, Dee Bradley Baker as the voice of Eagly, Sol Rodríguez as Sasha Bordeaux, Tim Meadows as Agent Langston Fleury, Elizabeth Faith Ludlow as Keeya Adebayo, Brey Noelle as Jaina Hudson/White Rabbit, Michael Rooker as Red St. Wild, Terence Rosemore as Otis Berg, Reinaldo Faberlle as Mexicali, Brandon Stanley as Ponyboy, Anissa Matlock as Titties, Dorian Kingi as Encino Man, Michael Ian Black as Cleavis Thornwaite, David Denman in a to-be-confirmed role, and Taylor St. Clair as Mrs. Chase. Guest stars include Merced as Kendra Saunders/Hawkgirl, Fillion as Guy Gardner/Green Lantern, and Sean Gunn as Maxwell Lord.
Gunn wrote all eight episodes of Peacemaker and directed three, including the first. Gunn, Peter Safran, and Cena serve as executive producers. Greg Mottola, Peter Sollett, and Althea Jones also direct. Based on characters from DC, Peacemaker is produced by Gunn’s Troll Court Entertainment and The Safran Company in association with Warner Bros. Television.
Peacemaker Season 2 is now streaming on HBO Max.