Right now Katello runs and is supported on both EL8 and EL9.
There is discussion about dropping support for EL8 so that Foreman 3.13 / Katello 4.15 would be EL9-only, and not run on EL8: Drop support for running on EL8 with Foreman 3.13
For that reason, EL9 may be a better bet in the long run.