This is Kevin Conroy's final vocal performance for a movie as Batman that was released posthumously since his death in 2022. The movie is dedicated to his memory after the ending credits.
The last performance of Mark Hamill as Joker, due to his retirement for voicing Joker because of Kevin Conroy's death in 2022.
The final chapter of the Tomorrowverse of DC Comics animated features, which started with Superman: Man of Tomorrow (2020).
The Teen Titans Earth being numbered 2003 makes sense; that's the year the show came out.
Constantine is the only one besides Spectre to be aware of the DCAMU.