You can't. That information is not anyhwere to filter. Itch does not know if something is just a demo or a chapter 1. And there is no free "full" game filter information either.
Even if developers would be inlined and able to accuratly give that information, they can't. A demo version can be "released" or "in development" just as well. Especially chapter 1 stuff would rather be released. And the other way round, just because something is "in development" does not mean it is unfinished.
There is https://itch.io/games/has-demo but this is for paid games that have a demo version. Itch has no concept of a whole project being a demo.