But that's just it. To find the games you want to play, shouldn't you search for what you want to play, instead of trying to avoid the rest? That's how the search system on itch.io is designed: to let you include tags. And it works very well. Many people find my games that way.
You're right, visual novels and horror games are very popular, so they tend to cluster in the first few pages of results. But again, they represent 3% and 5% respectively of the 900K games currently on itch.io (we've passed the 900K mark since a few days ago). if all you want is to exclude those, all you have to do is click through to page 3 of results or so. But are you really interested in all other kinds of games? Because if you're not, it's easier to look only at the tags you want.
Sure, you might have to run several searches that way. But I doubt you were going to find many games you like by looking at "everything but visual novels and horror". Since, you know, that would be 830K remaining titles.