Per Bitcoin Core 28.0 release notes (verified the hard way!):
The minimum required glibc to run Bitcoin Core is now 2.31.
This means that RHEL 8 and Ubuntu 18.04 (Bionic) are no-longer supported. (#29987)
I'm guessing they want C++17 features, seeing as they're dropping boost along the way. There may be a backport of a more modern GCC for ubuntu18 (I think it's on GCC8, there may be up to GCC11 for it). You can always check the backport repo I guess.
...all is well! Just can't run 28+
You can, as nc50lc has noted. You'd have to set --blocksxor=0 in your bitcoin.conf file.