When a vocabulary gets unwieldy (on one 15-year-old site with 27,000 nodes there are 19,000 tags) a first step in getting control is deleting the terms that are definitely not used. Rather than a dedicated module for deleting orphan terms, it makes sense to add the functionality to this module.
A person with permissions to the clean taxonomy dashboard should also have access to a "Bulk Auto Delete" tab where they can choose:
Needs review
2.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Spent way too long working on this but it's ready! OK i missed the "terms older than" capability— can make that another issue, this bulk delete / prune is a full feature in itself already.
Did a bit of refactoring of other code to make use of it here— the list page is now generated by a single query, it is sorted by usage count and then term ID now, and it's probably possible to make it user-sortable too.