I have a different understanding about the term "very useful" ;-)
I now know that it is possible on Steam. I have the tools to have it on Itch. Times I used it: about once on Steam for research.
Other people might want to use it more often, but I suspect they just want to filter out horror games, and this they already can do with a bookmark.
In the thread about exclusion of already owned projects, there is zero interest shown, and imho it is a much more useful feature https://itch.io/t/3477347/add-an-option-to-exclude-owned-games-when-searching
The reason why I do not use the feature of exclusion is the unreliability of tags. And my speculation why it is not implemented, is the lack of feature requests in combination with how hard it would be to implement in the current system.
an option of viewing by multiple tag inclusions
Inclusion?
Like this?
https://itch.io/games/newest/downloadable/exclude-jam/free/platform-windows/tag-...
You can combine and pile up "inclusions" all the way you like. No matter if tags, or technical information or whatever. If it is an url parameter, it will "include". And implementing negative inclusions on such a system sound like a hard task. It is optimised for intersecting sets.
Or is inclusion something different? English is not my native language. Do you mean a logical "or" maybe?
Like, give me (platformers or shooters) and robots. Including the searches of platformer & robot with shooter & robots.