If you include stuff like Visual Studio autocomplete and looking stuff up on Google, you'll probably get to 95%+ for games being worked on within the last year. If you include AI-generated code in libraries and game engines, you'll get close to 100.

Besides, it's usually not the devs posting here. Random users finding a free game will have to rely on the information that is available. Even the Steam AI disclosures don't include any information about the game's code.