Oh, there are content sites with exclusion of tags, but specific to gaming it seems rare, except for the market leader. One would think that many would imitate Steam for that, but apparantly not.
Might have to do with the catalogue size. Of the other sites, Steam has the biggest by far, yet they only have ~160k titles compared to the ~850k titles on itch (560k claim released status). And of course, the other sites usually lack the tags to begin with.
I believe Steam just added it, because it was dirt cheap to add into their existing framwork of tagging. It starts with them having only a limited selection of tags and when filtering their catalogue by tags it appears as "tag" and "-tag", so basically, even the exclusion tags are positive searches.
In contrast to "filtering" on itch, that is done (or appears) as added sub urls. My guess is, that there is not really filtering done at all. It is a cross-section of overlapping url conditions. And there just aren't nightly builds of negative tags that could be overlapped. There would have to be built two lists for every tag, and since tags are custom, one does not just build a list of 1-million-minus-the-seven-games-with-obscure-tag to overlap ... for every obscure tag.
So starting at my guess, filtering would not be integrateable into existing browse, but would have to be added on top. For singular that was trivial. For multiple, it apparantly is not.
But it might be a help to many, if the existing option would be available as an UI element in browse. I would not need it, but that is because I do not put too much weight on tags anyways. Dev tagging is often horrible. Tags on Steam are user chosen by majority. At least it says "Popular user-defined tags for this product" under a game.