See the part about headache ;-)
Manually compiling your collections into an ignore css already works. But see above. If you keep dynamic loading on, there seems to be trouble with that method of filtering. Also, I really think, filtering could be done with the number of the game. It is a number that is counting up and is currently about 2.5 million. That many projects exist or did exist on itch.
Edit. Actually, with the filtering, since it would only hide and not replace it with new items not filtered, it might be better to overimpose a transparent image or some other style that highlights or rather "greys out" the game, so you would still have access to it and see it appear in the browse list, just not as prominent. While you might have it in your library, even in a collection, you might have put it there and forgot it. Maybe a later option for customizing the way the thingy does it's thing.