Just FTR - git /can/ store that information, but it requires human input.

If you rebase the feature branch into the main branch THEN follow it up with the merge commit that records the branch name you store the branches (that have been made a part of main) and can see where they are in your log

Mercurial's notes can become cumbersome if there are a large number in the repository, but, obviously, humans can sort that out if it gets out of hand