Interesting. Since I tried doing similar a few years ago, I might give some feedback. Or ask stupid questions, since I have trouble reading js and it has been years ;-)
When stress testing mine, I ran into storage limit. You might want to ask for unlimited storage permission. It is something like 10MB and Itch has 2 million projets. With 200 bytes per entry, you might run into problems for users that put a lot of items on ignore.
Do you load the "data" on each page load from local storage? If so, that's not good. Then again, as I said, I have trouble reading and understanding js. When your database gets big, you will have a seconds delay. I think you circumvent this by just waiting and then doing your apply if needed. I switched to just querying the items that are on a page and new items, since the storage access is highly optimised.
There is a 1.5 second recurring timer. Is that for endless scroll or something? Or for reapplying changed rules when on another page? I initially needed the important! thingy only for endless scroll, but when I changed to a per case query, this was no longer needed.
But very interesting how different approaches go. My initial motivation (a now deleted thread) was to mark owned games. If you have a library full of owned games and collections of known games, and see them where you regularly browse, that is clutter. So I needed to be able to mark games inside the library without hiding them. So my approach is to be location sensitive. With a 2 fold css. One for library and one for regular browse pages. Plus it is 2 lists, so you can have an ignore list and a highlight list. It also solves the problem how to unhide games when hidden: just turning it "off" renders the hidden games with the library css, so they are visible again and can be toggled.
I also have a hovering button like the add to collection button ;-). It did not make it into the tampermonkey script. I sometimes wonder, if I should update it with some bells and whistles. But after two years of exposure, I can rather confidently say, compared to how much people ask for exlusion features and whatnot, there is not much interest in client side solutions. They want an Itch side solution. And at least for exclusions, there seems to be coming one. The "hide my collections" or "hide things I do not want to see again" function would be next on the wishlist. Steam introduced an ignore button at some point. And at least it shows on a thumbnail, if you have the item in your library or in your wishlist.
Maybe your's gets more exposure by being in the Chrome store.