It seems like some maintainers prefer waiting with upgrading because cabal-install for GHC 9.12 still has issues: Release a hotfix for `cabal-install-3.14` · Issue #10836 · haskell/cabal · GitHub
Just putting this out there in case somebody prefers to wait until the toolchain has stabilized.
I have added unofficial GHC JS bindists: GHC JS 9.12.2 by hasufell · Pull Request #310 · haskell/ghcup-metadata · GitHub
In case others wish to try it: llvm 20 on 9.12.2 passes my smoke tests. To have it work with ghc 9.12.2 you have to edit the configure file and change the line “LlvmMaxVersion=20” to “LlvmMaxVersion=21”, then run configure, then sudo make install. I have filed, #25915: ghc 9.12.2 won't use unsupported versions of llvm · Issues · Glasgow Haskell Compiler / GHC · GitLab, for the issue that you can no longer use unsupported llvm versions as you could in the past.