It needs to be properly implemented
It does. But to implement it properly, is not not a trivial task. Implementing filtering one tag was trivial. Multiple exclusion would not fit in the existing frame work and it would need a big overhaul.
Also, many games just tag horror or visual novel, but really are not. Since the genres are so popular, devs tag it, if the game even remotly could be called such.
Only by reading those discussion I realized, that steam does has a tag exclusion. I could not find such a feature on gog. Humblebundle does not seem to even have tags. Origin/ea seems to only have positve filtering. I could not find a filtering on epicgames. If google play has tagging and filtering, it is well hidden.
Seems to me, that tagging itself is a minority feature and I have yet to find a game store besides steam that offers negative filtering.