You are correct. Ignoring a single tag is not hard. It was implemented quite rapidly years ago. Here you go
https://itch.io/games?exclude=tg.visual-novel
.
?exclude=tg.visual-novel
There is a reason why that year old thread is named excluding multiple tags ;-) There is speculation why this is not been done, but since there is a feedback button, I assume the feature is not that high in demand or too costly. You need not only the feature for multiple, but a user interface that will also work on mobile.
Maybe you could make a custom css, that helps put the single filter in the url.