I agree that "horror" in general is not something that should be considered kid friendly, while also not being strictly adults only. Maybe a third category, not intended for kids would do good. But all this is self information from amateur publishers and thus unreliable. Also standards what is not suitable for a 16yo differ greatly worldwide.
Personally I am not bothered much by horror games, for three reasons. I do not regularly browse popular on itch. I know how to filter them, if I wanted to. And if I can scroll past advertisements, I can scroll past creepy stuff. The cookie warnings in the net bother me more than advertisments.
It is a bit like the chicken and the egg question. Does itch promote horror? Or is horror popular and thus featured on itch. I would go with, it is the type of game that does not get featured much elsewhere, but is still popular enough that it will appear immensly popular where it can be easily published. Devs do not have to go through whatever process games go through on Steam to go from wishlisting to published.
Could also be a side effect of "horror" being popular on youtube. Or popular enough to skew apparant popularity on itch.
The most practical way is to indeed include tag exclusion, which is possible only through editing URLs rather than through the actual website.
While this works regardless of any maturity settings made by the developer and for tags that are not even covered by any maturity considerations, it is unwieldy to use. I even made a little tool to help with that, but that does not work for everyone and in the end it still depends on accurate tagging by the devs.
So even if you exclude horror, you will still get psychological horror, creepy, survival horror, zombies and so on - if horror was not also tagged. And of course, you will also not see the games that you would not consider horror but are still tagged this way.
Well, to me, horror is a genre anyways, and because of that I do not agree with most horror game tags. A jump scare might be unnerving, but horror it ain't.
Maybe the situation would be better, if the recommendation page would be better, so people would browse there more often, instead of browsing popular section.