(also discussed at Phabricator)
There really should be a way for users with the (changetags) permission to be able to remove tags from deleted revisions. This would be especially useful for sites where pages in certain namespaces can't be undeleted once deleted.
There are some sites out there that have restricted the ability to undelete pages in certain namespaces, but haven't restricted the ability to delete pages in those namespaces.
Not being able to remove tags from deleted revisions is a real problem for me. As one of the wikis I administrate have 8,716 deleted revisions to remove a specific tag from before the software will allow me to remove the tag from the remaining 5,000 revisions.
If there are any methods that one can use to remove tags from deleted revisions, please let me know so that I can work towards removing the tag from 8,716 deleted revisions. And then I'll be able to delete the tag altogether.
If I'm not mistaken, it might be possible to do this via Special:ApiHelp and/or Special:ApiSandbox. Though the few methods I've tried so far either haven't worked. Or I'm doing them wrong.