In haskell/core-libraries-committee#146 a number of modules (based on @bgamari's spreadsheet were documented as unstable.
-
haskell/core-libraries-committee#146 (comment) has the exact list of modules.
-
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/10422 is the implementation PR
However, these modules were not formally deprecated. I am not sure why this didn't happen.