I briefly thought about trying to make an extension. For that I tried to do it by hand. Like saving a collection and extracting the links somehow. If I can do that manually, it should be a matter of automating the process.
First I noticed that you need to have infinity scroll enabled to even see the whole collection - and possibly scroll it down. The content seems to be generated on the fly by javascript. And for some reason I could not save the webpage. Only as a full html with image download I got the html to parse later, wich I did not try so far.
There might be more elegant things to filter for, but the link to the game seems rather unique. So what is the regex for a link to an itch game.
"\"https://[-a-z0-9]+.itch.io/[-a-z0-9]+\"" or something similar.
So the extension would grab from the current page and populate a list, and present it to the user to verify and maybe edit, and then save to the ignore list.
After that, bells and whistles, like give ability to igore single game by providing a button similar to the add to collection button already present in browse for each game if you hover on the thumbnail. And of course, editing the list