Did you use the feedback button and select request feature and, well, requested the feature?
You are the third person I am asking this. All three of you opened such a thread. The first one refused to request the feature, the second did not answer yet. My current conclusion is, that this feature is not as much in demand as some people think it is.
And I am also asking for a use case how the feature would benefit a search - and was not answered yet. Although, it is browse we are talking about (the section where you can select tags). You cannot use tags in a search. Filtering searches by a tag subset would be a different feature.
The use case would be how to find faster or better the game you are looking for.
Your use case seems to be, you just do not want to see shooter games. But there are problems here. shmup is only used by like 10k games, compare to 80k for shooter games. How accurate do you think the developers of such games used the tags to describe their games? So you might still get a loot of shmup in those 70k games that are tagged shooter, but not shmup. Or the other way round, you might filter out your new favorite game, just becaue the dev considers it shoot em up, while you would not.
The tags on Steam are by majority consensus of players. In combination with a lot of users, tags there are somewhat accurate. And there is a very limited pool of tags. Filtering is both easy to implement and will be rather accurate there. This is why I think Steam has it.
Personally, I use individual filtering. I do not blacklist tags, I "blacklist" games. This is also very helpful for games in your library.