You want to know if Red Hat Enterprise Linux (RHEL) 9 is supported by DLP 15.8 and 16.0 before upgrading the RHEL servers.
Release: 15.8 MP3, 16.0
RHEL 8.7 is the highest version supported on DLP 15.8 MP3.
RHEL 8.8 is supported on DLP 16.0 MP2 and higher.
RHEL 9 is not supported with DLP 15.8 and 16.0.
Support for RHEL 9 on the Enforce and Detection Servers is planned for the next major release after 16.0