Siren-X and Vibe are the only known Earth-X doppelgängers of main Arrowverse characters who didn't appear in Crisis on Earth-X. Her absence on the crossover was because Katie Cassidy was mourning the death of her father, David Cassidy, during its filming.
This is the first appearance of Earth-X since the "Crisis on Earth-X" crossover.
DeVoe calls Siren-X "banshee". In the comics there is a Superman villain called Silver Banshee that has powers similar to Siren-X's.
Cisco says he's had his fill of "nuclear bombs this year", while discussing what to do with Fallout. This is a reference to the episode Enter Flashtime (2018).
Cisco mentions that Fallout is so far off the grid that it would take Jack Ryan "two Tom Clancy novels to find him", referencing the former fictional character by the latter author.