The henchmen in the karaoke that send the milk over to Batman are dressed in the style of the 1960s Batman series Catwoman henchmen
During the bar karaoke scene, the henchman that is dancing behind Batman is doing the Batusi from Batman (1966), possibly as a tribute to Adam West who died earlier that year.
Min and Max who originally appeared in Two-Face: Part II (1992) apparently survived their gunshot wounds they received at the end, here they are seen alive and well singing karaoke at the bar in Bludhaven.
When Booster Gold calls Batman after the karaoke bar brawl, the ringtone resembles part of the main theme of Justice League Unlimited (2004), in which Booster Gold is a member of the expanded Justice League.
Melissa Rauch has taken over the role of Harley Quinn because Arleen Sorkin retired from voice acting entirely.