Reverse-Flash was one of several Arrowverse characters that failed to show up in "Crisis." Pyscho Pirate, one of the characters from the original comic, was also expected to make an appearance but was also scrapped.

Why was Reverse-Flash not in Crisis?

Crisis on Infinite Earths' Marc Guggenheim explains Reverse-Flash's non-appearance, saying it's due to not being "beholden" to the original headline. ... Thawne was the central antagonist in season one, and appeared quite a few times afterward, including Legends of Tomorrow and the Crisis on Earth-X event.

Where was Reverse-Flash during Crisis?

In the original timeline, the Crisis took place on the night of April 25, 2024, where the Flash fought his nemesis, the Reverse-Flash, with the help of his allies in an extreme street battle in Central City.