evidently the one solution, adding "?exclude=tg.(tag)" doesn't even work right now, at least on my end. it's been way too long for this to be addressed in a real capacity. I had assumed all the games were simply improperly tagged, but no, if you filter to include only horror, the same games show up.
That is something on your end. That url below will give you no results.
https://itch.io/games/tag-horror?exclude=tg.horror