I found
a Phoronix article about RX 6000 support on GNU/LinuxIt looks like the issues are the same for every architecture, not only POWER9
Some things are already done, for example, the kernel 5.9 is in Fedora 33. Debian stable-backports has 5.8 but sooner or later it will get 5.9.
The other things mentioned in the article (LLVM, GFX, Mesa) appear to be available in Debian unstable so it may be possible to simply run unstable or backport them to buster.
People can actually upgrade to Fedora 33, upgrade to Debian unstable or start running the newer kernel and/or preparing backports of these things before getting the RX 6000 series cards. So you can see if everything else on your computer works normally with the new kernel and libraries before spending money on hardware.
Once you have all that software installed/upgraded, you may simply be able to insert the card and it works immediately, whether you are on x86 or POWER9