The X-Men will join the Marvel Cinematic Universe, but it might not be until Phase 6 of the MCU. The Disney/Fox merger brought a lot of titles and characters to the Mouse House, among them the Fantastic Four and the X-Men. The mutants’ storyline recently came to an end with X-Men: Dark Phoenix, so the characters are ready for a fresh start in the MCU.
During the Marvel Studios panel at San Diego Comic-Con 2019, Kevin Feige confirmed the X-Men are coming to the MCU, but didn’t give any details. Prior to this reveal, Feige unveiled the slate for the MCU’s Phase 4 and teased two projects in development that could be part of Phase 5, but it’s unlikely that the X-Men will be included in it, with Marvel instead waiting until Phase 6 at the earliest.