I recently encountered this question on MSO, where a user asks about why certain tags appear in the tag overview page.
This issue is caused by a synonym that redirects users to a non-existent tag.
I've created this SEDE query, and on Stack Overflow alone, a total of 60 approved synonyms exist that point to tags that don't exist. On other sites, I expect orphaned synonyms will exist too.
These synonyms are still functional: they show up in the tag page, when searching for it you still get redirected, and if you try to use it in a new question, you will get a prompt if you want to create the synonym target (if you have enough rep to create tags). Also, if the synonym has a very similar name to the target tag, attempts to create the target tag will fail with a confusing error message.
Could we destroy all synonyms too when destroying tags? Or add a separate cleanup script for synonyms?
Not sure if should be a feature request or bug report, feel free to retag.