The question Understanding risks of logging into Stack Exchange sites from a shared computer or network has two tags which have only been used once and have no tag excerpt or wiki: shared-devices and shared-network. According to What causes a question to be tagged as [untagged]? that means they should be deleted after six months:
Tags that only exist on one question are automatically removed after 6 months, unless the tag has a tag wiki.
They should have been removed somewhere halfway 2019. Yet, the tags still exist.
Looking at the revision history, they've been removed by a ♦ moderator (which presumably deleted the tags about 24 hours later) but that edit was rolled back after a week. My gut feeling says this somehow blocks the automatic pruning of these tags, but I could be wrong since of course I have no way of looking at the source code.
I've checked Why wasn't this tag removed - despite having single occurrence? but that seems to be a different case; the tag there has an empty tag wiki and excerpt: https://mathoverflow.net/posts/295234/revisions and https://mathoverflow.net/posts/295235/revisions. Those IDs can be found with this SEDE query, and the corresponding query for shared-devices and shared-network comes up empty.