I know that if there's only one question with a tag, it gets deleted and the question becomes
What if the tag has a wiki? Does it get deleted too?
What are all the ways this can happen (a tag being deleted automatically that is)?
Tags that are used a single time and are at least six months old are removed monthly. (This leaves the question "untagged".)
Tags that have no questions are removed daily.
Wikis are not removed with their tags (which is why we have a tool for "orphan" tag wikis).
Note also that when a question is migrated, if the receiving site doesn't have any of the tags on the question it will also arrive there "untagged".
Of course, the Stack Exchange developers can step in and remove a tag, but that's a manual process.