This is not an easy question to answer. The latest I have heard (and tested) is that:

• Support for RHEL 9.x should ONLY exist if you
  • install the product on RHEL 8
  • patch it to P05 (or higher)
  • then upgrade the server to RHEL 9
• If you install from scratch, you need to use the "base install" (which is on P02, and this version is not supported on RHEL9)
  • Which in practice doesn't make IG&L compatible with RHEL 9.x
  • Although plans are in works that the base install will be bumped to P11, and THEN the newer RHEL 9.x will be supported