In general, I do not see previous CUCM JTAPI versions being tested with newer OS/JVM combos - it pretty much is what it is at the time they were released. I would have expected an update to this page for CUCM v14, however (where RHEL 8 support would make sense as the current latest release)...let me see if I can get any updates to the page published ASAP.
I would note that I would fully expect v12.5 JTAPI to work just fine on RHEL 8, as the JVM is the main concern.