I’m not sure if this counts as a feature or a bug — I think it might be a “defect” in the technical sense. Or maybe I’m just misunderstanding something. 
Tag synonyms are a useful feature for keeping a site organized. But, right now, if you create one, the tag remains but is shown as having no associated topics. This means it stays on the tags page, and comes up when people are searching. The latter behavior might be desirable in some cases (like, if you have a bunch of specific terms mapped to a more general tag), but in other cases (people keep creating a typo tag, you are tired of cleaning it up!) it’s definitely not (it should only come up if that exact typo is used again).
Second, if one hits the “Delete Unused Tags” button, all of the synonyms are destroyed. That can’t be right, can it?
Maybe one approach would be to automatically add these to a “tag synonyms” tag group, and then handle that group specially?