plan.json doesn't include compiler id.

[phadej]

Since some version of cabal-install, the locations of ghc-specific store take "project unit id" (e.g. ,("Project Unit Id","ghc-9.10.1-69c3")) into account. So it's not store/ghc-9.10.1 but store/ghc-9.10.1-69c3). However this information is absent from plan.json. plan.json contains just a version as compiler-id:

% python3 -m json.tool dist-newstyle/cache/plan.json|less
{
    "cabal-version": "3.14.1.1",
    "cabal-lib-version": "3.14.1.1",
    "compiler-id": "ghc-9.10.1",
    "os": "linux",
    "arch": "x86_64",
    "install-plan": [
...

Therefore tools (e.g. cabal-plan) which traverse the plan.json but also information in the store (e.g. license files), cannot do this anymore without also somehow finding the corresponding GHC (which can be anywhere, that's the point of an additional few bits - to disambiguate different GHC builds even with the same main version).