I ran into (kinda) the same problem on this wallpaper https://wallhaven.cc/w/dgdv8j when I tried to add the "bracelet" tag to it. The correct one for now is "bracelets", and it has "bracelet" as its alias. For most tags, when you type an alias in the "add tag" field and hit Enter, the correct tag is added instead. However, when there existed a tag that has since been deleted, this "alias" feature doesn't work, and the "This tag has been deleted and can not be created again!" message is displayed.
Seems like this should be pretty easy to fix in the code: just change the order of the checks. If the text entered is an alias for some tag, add this tag instead. THEN check if the text entered is some old banned tag and display the error message. THEN start the "add new tag" routine if it's something new.