There is many free as in freedom (known here as open source) games. It would be very suitable to have a way to find such games. It can be a special tag, or, may be, a search filter.
Way to search libre (open source) games
Oh, that's great!
But what about more general method of search?.. I am not shure, that I will remember this urls with assets, and I am afraid some other users too. Also I try to go to https://itch.io/games/code-gpl and got 404. Than I have got that I should search code-gpl2 and code-gpl3 but it is not seems too clear, isn't it? I mean I just want to explore games with any free licenses (or, may be, with few of these at once).