On T244368, we added deletion of old wmf branches in mediawiki/tools/release/delete-wmf-branches.
This has since been superseded by @mmodell's e641ec26 - "Add the very dangerous --delete argument to mass delete branches" in branch.py.
That feature should additionally do the equivalent of git merge-base to find a branch point and tag releases (or at least a close approximation) before deleting the branch, so that users and tooling can reference it without the costs of keeping the wmf/* branches around indefinitely.
Pairing with Tyler today yielded a gitiles URL:
Finding the parent of that commit should be equivalent to merge-base.