There is! Creators can indicate a license for their code and/or assets in their game metadata, and you can search for it with a URL of the form https://itch.io/games/code-mit or https://itch.io/games/assets-cc0 (the latter is poorly advertised).
Viewing post in 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).