RHEL 8

Yes, while Oracle 19c is supported on RHEL 8 now, please now that only Oracle 19.6.0.0.0 (JAN2020 DBRU – Patch 30557433) or later applied as well.

$ export CV_ASSUME_DISTID=OL7
$ <19c OH>/runInstaller -applyRU
Installation on red-hat 8 worked, there are some issues
1 Like you said, the base release does not recognize red-hat8, so we still have to do the following work-around: export CV_ASSUME_DISTID=OEL7.6
2 AFD does not recognize Red-hat 8

$ORACLE_HOME/bin/asmcmd afd_configure

ASMCMD-9520: AFD is not ‘supported’

3 there is a major issue with openssh 8.8 causing scp to fail during install, for this reason some prechecks are not working. There is a work-around in document.id Doc ID 2555697.1 (rename scp temporarily)

I would expect Oracle to deliver a base version of 19c that includes those fixes