As a NixOS user, I have in the past found that I need an older GHC
release that is no longer available (or not yet available) in nixpkgs.
While mpickering's old-ghc-nix  conveniently allows standard GHC
binary distributions to be adapted for use under NixOS, I have found
that it is less than reliable when building later projects .
For this reason, I have started collecting a set of nix expressions for
building GHC versions all the way back to GHC 8.4.1 and extending up to
GHC 9.0.1-rc1. In addition, there is a binary cache with pre-built
binaries (complements of Cachix) here . I intend on continuing to add
new releases to this as needed.