It's been frustrating for me too, not knowing what to tell people for such a long time, but I trust the dev team. If they weren't able to do it in years, there must be a good reason. Edit: in all this time, people continue to demand and assume bad faith, which is doubly frustrating. There are many other features on itch.io that can help us discover games, but people won't use them because they've decided that the one thing they want is tag exclusion and nothing else.
If people assume bad faith, they should apply Hanlon's razor. Also, if I were to believe the threads about tagging and exclusion, I would also have to assume horror is unpopular. Well, horror is very popular on itch, so maybe tag exclusion is not in such a high demand, as those threads might suggest. After all, the people happy with recommendations and related games and other means of finding games do not regularly make community threads about how things are ok. It is the people fed up with horror stuff that do.
Sure, it might be a handy feature in some situations. But would it be a feature that most users would use? Or is this one of those scenarious, where the people able to use that feature would not need it, cause they would know how to find stuff without it.
I have trouble believing someone looking for the 10k new games in the last month. Like, itch show me everything, except horror and visual novels, I will dig through the remaining 8k games myself. To be realistic, you have to start somehow with positive filtering. And this should thin out the games already. And who knows, maybe that puzzle platformer pixel adventure you might like, does has some minor horror elements that made the dev chose horror tag - after all, horror is popular, so everyone and their dog is tagging it.