I am not exactly promoting it, but for anyone willing and able to use custom browser extensions and tampermonkey scripts, there are two working client side solutions to two wanted features on my project page https://redonihunter.itch.io/itch-browse-tag-exclude
The search mods are capable of removing horror and visual-novels at the same time. This is possible, because vn are a genre. And of course there is this old feature of adding a search query to exlclude a single tag. The extension adds a box to comfortably use that exclude feature. And the tampermonkey script adds buttons to exclude any of the genres.
And as a bonus I uploaded the bare bone tampermonkey version of the exclude games feature to the project. The script features two lists. Any item on a list will by modified by a css style. Like display: none to hide (exclude) the item. It can switch to an alternative style for both lists. The alternative style is used on library pages or when toggled. You can add items or developers to the lists. And you can export and import the lists.
So the use case for the feature in this thread, just add all the owned games to list 1 and that's it.
It uses keybindings and you need to hover your mouse over the item in a browse view like library or search or browse. Default is key 1 and 2. 3 and 4 will add not the game, but it's developer. And 5 switches between the two styles.
Use list 2 for things you already know or you want to ignore or whatever. You can also export your owned games and clear the lists to do some special browsing sessions with ignore lists and later reimport your owned games. Also you do not need to ignore things. You can highlight them. It is just css styles. Stuff like scale: 0.5 does work too. Configuration is done by editing the tampermonkey script's constants near the beginning of the script.