Selina Kyle calls one of her pet cats "Eartha." This is an in-joke/reference to Eartha Kitt, who played Catwoman in the 1960s Batman TV series.
This is the first time both Peyton Lists have worked on the same project. Peyton List as Batgirl, and Peyton List as Poison Ivy, a role she previously portrayed a year prior on Gotham.
Both Kevin Conroy and Mark Hamill had expressed interest in taking part in a "Batman: Hush" movie as Batman and Joker respectively, despite Hamill initially considering to retire from his role of the Joker after Batman: The Killing Joke (2016).
Nightwing refers to Riddler as a "C-Lister" despite the fact that, historically, the Riddler has always been regarded as one of Batman's major enemies.
The comic story was rewritten to fit into the continuity of recent DC animated films. For example, Damian does not appear in the 'Hush' graphic novel and Luthor was actually president of the United States during these events.