Thanks for the feedback, everyone! :)

Since option (C) received the most support, I opened an MR to implement it:

https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3154

Once it is merged, I will make an announcement of this change, as
requested on r/haskell:

https://www.reddit.com/r/haskell/comments/g3glwn/migration_path_for_last_c/fnrfzf1/

Cheers,
Simon