ROCm/ROCm#1367 (comment)
The canonical documentation site (and I say "canonical" because there are scraps of conflicting ROCm information all over the internet), is presumably this one. However nowhere is it made clear in this documentation, and it should be in BIG RED WRITING, that on the most widely deployed desktop version of Linux, by far, ROCm doesn't install.
No point in having installation documentation which merrily tells you what needs to happen to get ROCm up and running, when there's an obvious issue that hasn't been resolved. This Kernel 5.8 problem, which is the default kernel of the current Ubuntu LTS, should not be buried as if it doesn't exist.
Mentioning in brackets as you do "(Ubuntu 20.04.1 (5.4 and 5.6-oem) and 18.04.5 (Kernel 5.4)" is insufficient because most users will not even see this, nor indeed do many users even care what kernel they're on, if they even know. They just expect a virgin 20.04 + ROCm 4 installation, to work. If it doesn't please say so boldly, and indeed, if it doesn't work on Linux's "baseline" distro, then it shouldn't even have been released. This is not some fancy Fedora cutting-edge "testing" distro. This is Ubuntu LTS.