(As an addition to Fixing the sucktastic merge UI for tags and Merging / synonymising should warn of tag wiki loss)
Currently, there's no way for a moderator to undo a tag synonym / merge. While a moderator's intent behind making a synonym mapping could be well-intentioned, sometimes what happens is that better arguments get put forward in meta / chat on why the synonym shouldn't exist.
What would be nice to have is for StackExchange to have a way of undoing this action. Yes, a mapping can be deleted, but it does not restore tags for merges. If there's a way for the system to maintain a database of tags and allow a grace period window for tags to be un-merged, that would be amazing!