There does not appear to be an explicit conflict policy for EPEL8:
https://fedoraproject.org/wiki/EPEL/FAQ#Does_EPEL_replace_packages_provided_within_Red_Hat_Enterprise_Linux_or_layered_products.3F
I got a report against python3-s3transfer and python3-botocore
conflicting with the CentOS 8 HighAvailability repo. No idea if this is
an issue or not: https://bugzilla.redhat.com/show_bug.cgi?id=1821630
It looks like we have avoided conflicts with the "ha" repos in the past,
and I can enable the rhel-8-for-x86_64-highavailability-rpms repo on my
RHEL8 developer license machine so it does seem available to everyone
When EPEL-8 was getting set up, HA was a paid add-on for EL8 and so not available in the developer license. EPEL therefore did not conflict against it or use it as a 'you can't build this in EPEL'. EPEL will end up conflicting with some RHEL channels.. but since there is no 'Requires: Conflicts:' in repo land we can't do much about it.