If you navigate to the last page of tags on basically any site, you'll see pages of tags with zero questions. Doing a little investigation (clicking on any of them) shows that they are all synonyms of existing tags (e.g. tag-editor-badge is a synonym of edit-badges). All of them however, are shown as being on 0 questions.
However, from this:
Tags not associated with any question are automatically destroyed at 03:00 UTC every day.
Yet this doesn't seem to be the case when the tag is a synonym of another one. Why not? If the reason is that it removes the synonym if the tag is destroyed, could they at least not be shown in the lists of tags?