As far as I'm aware there's no built-in way to specify if a project is complete, so there's likely no way to search for completed ones. This could be something you suggest to the itch.io developers tho
Viewing post in Finished games
This could be something you suggest
It is all there, but this is not enforced or curated and we are talking about mostly amateur developers and small time indie deves here. You will find finished games that have status in development. And released games that get content updates. To make it more complicated, in the hover screenshots, in development for paid games is called early access.
When is a game finished? There is not even an agreed upon version numbering. There are devs that use 1.03 version for unfinished games that are less finished than dev's games that use 0.1.3
One can guess from the description, but searching from the outside is unreliable.