It would seem was unable to understand the purpose of the tool, I did try looking for mentions of how it's supposed to work in the description but maybe it's my dyslexia but I never found it. I ended up just using https://better-itch-search.kalrog.com/ which is quite powerful and I can recommend!
(Keep in mind because it's just a scraper and cataloger it isn't always as up to date as the actual site, though I feel the curator should be the one to say its limitations and I'm sure they did somewhere.)
Viewing post in itch browse tag exclude comments
I should rewrite the description some day.
I guess what most people really want, is to exclude horror games or visual-novels and all it takes for that, is a bookmark. No need to install anything or visit external sites. The usual complaint would be, that 90% of the top pages are horror and visual-novels, and thus people asked for tag exclusion.
But from my browsing experience on Itch, exluding tags is not actually gonna solve this. Or all the clutter appearing again and again at the same places you look every week. You also can't exclude your library or known games by tags.
So imho the better solution is this https://itch.io/t/1018893/a-never-show-me-this-again-please-button
And that's what the tampermonkey script does. You can mark&hide your whole library, any developers you dislike, and all games you already have seen, at the press of a button.
You could even actively search for a tag you dislike, and mark all the games. For experiments like this, I would recommend using the save and load functionality of the script. So yeah, one can emulate fully working multiple tag exclusion with that script. But why bother, if you can just weed out games occasionaly at the places you browse regularily.