How much effort does backwards compatibility require from library authors?

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Evidently nixpkgs is stuck on 9.6 due to “significant breakage”. It seems close though: haskellPackages: Stackage LTS 22 -> 23; ghc: 9.6.6 -> 9.8.4 by sternenseemann · Pull Request #371032 · NixOS/nixpkgs · GitHub.