That's quite interesting: despite the official documentation, which says that only GHC >= 7.10 are supported, one of CI jobs installs GHC 7.4.1, using ppa:hvr/ghc. I don't know whether this configuration is used in the wild, it might be.

despite the official documentation, which says that only GHC >= 7.10 are supported, one of CI jobs installs GHC 7.4.1,

setup is "conscious" about certain GHC versions (those listed in https://github.com/haskell/actions/blob/main/setup/src/versions.json), but you can ask it also for versions it is not conscious of (e.g. 9.4.3 atm). It will do it's standard job then. It won't resolve 9.4 or latest to 9.4.3 because it does not know 9.4.3, but if you ask fo 9.4.3 directly, it will try to install it.

Ubuntu-20.04 will only support a few legacy GHCs: relevant are 7.4.2 7.2.2 7.0.4 (and maybe 7.10.3). https://launchpad.net/~hvr/+archive/ubuntu/ghc?field.series_filter=focal