I know I can check the tags on my Game User Tag Votes, then I open the store page of the game that I've added the tag and remove it from the user-defined tags menu.

I added a tag to The Elder Scrolls Online - Morrowind when the DLC first released, now it's no longer in the store since the DLC got merged with the main game, If I check the DLC on steamdb https://steamdb.info/app/582880/ and open the store link it just redirects to the main store page.