Oh it fits the topic. It is about managing the games to play. This includes saving them in collections for later, managing your favorites and finding new games to play, of course.
Maybe have a look here https://itch.io/t/3477347/add-an-option-to-exclude-owned-games-when-searching I have no idea why that thread has 8k hits. Probably scam bots posting there. Or a few people going there often to copy the code examples.
Excluding games you already have on your collection(s) is a nice idea. I thought, this is a thing I could do client side, and so it was. Was a fun learning experience and after learning about browser extensions, I thought, hey, it should be simple to make a button for the existing exlude feature that does exist. And so it was. Later someone inspired me that filtering genres would be super trivial and so it was. And I tuned down my extension for game filtering till it fit in a tampermonkey script, so it reaches more browsers. It will do collection filtering if you add your collection to the filter list.