To Reproduce
Run an API query on tag-synonyms, you will get a result that includes something like this:

Issue(s)
Click on the applied_count link and you get redirected to this page which is a 404.
It isn't quite clear what applied_count is anyway. The docs don't explain it, and on the main site synonym page it is called 'Renames'. Any chance that could be fixed while you're at it?