How do you come to that conclusion? I see quite a few backports and fixes on nixos-19.09.
@schmittlauch I use the nixos-20.03 branch for haskell related stuff and do backport fixes, like in [20.03] haskellPackages.binary-strict: 0.4.8.4 -> 0.4.8.5 by erictapen · Pull Request #82780 · NixOS/nixpkgs · GitHub.