Burt Ward says "holy crimson skies of death" when he sees the antimatter red skies, a reference to how his character of Robin would say a phrase that started with 'Holy' in the 1966 Batman series. He's even wearing the same sweater (the sweaters colors are the same he wore as Robin) his character as a civilian wore. The name of the earth is Earth 66 a reference to the year that the show came out in 1966, the theme from the show also can be heard. He's also shown walking a German Shepherd a reference to Ace the Bat-Hound who was the same breed of dog in the comics.
A character presumably Alexander Knox is holding a newspaper at the beginning which reads, "Batman stops Joker", is played by Robert Wuhl who played the reporter in Tim Burton's Batman (1989) which also featured the Joker. The Danny Elfman score is even heard playing, the name of the Earth is Earth-89 a reference to the year the film came out.
"Star City 2046", the dystopian future that the team had accidentally traveled to, is, according to multiverse readings, actually exists on an alternate Earth dubbed Earth-16, which was where baby Jonathan's pod was accidentally sent. The Earth number is a reference to the year the Legends episode "Star City 2046" aired, namely 2016.
Katie McGrath couldn't be happier about being part of the Crisis on Infinite Earths crossover event "It's been kind of nice because every year I'm never involved, and I've always felt a bit left out because it's always such a huge thing and a big deal and kind of very special because obviously so much of everything leads up to it," McGrath told TV Guide. "And to be involved in it this year I was like, 'Oh, I kind of feel like part of the gang.' I felt very flattered actually... It is insane what they have managed."
The Monitor's opening narration is almost exactly the same as the one that explains the DC multiverse on the very first page of Crisis on Infinite Earths by Marv Wolfman and George Perez.
Burt Ward: A month after his cameo in this episode, Burt Ward received his star on The Hollywood Walk of Fame next to Adam West's.
Robert Wuhl: A montage showed the destruction of several of the Earths in quick succession, starting with Earth-89. Then, it jumped to San Francisco on Earth-9. A tight shot of Hank Hall's Hawk, as played by Smallville alum Alan Ritchson, squints into the bright white light, while Jason Todd in his Robin uniform looks on in horror. Rumors of a Titans cameo started circling months before the crossover aired in August, but it was recanted shortly after it was reported. Another report in October doubled down on the Titans cameo and accurately described both Burt Ward and Robert Wuhl's cameos.
Alan Ritchson: was called to reprise his role as Aquaman from Smallville, but he couldn't due to scheduling difficulties from his role in Titans (2018) and due to the complications of going to the other set. Ritchson makes a brief cameo as Hawk via archive footage, along with Curran Walters as Robin.