Like the official site about Deck verified requirements??
The title should have full controller support, use appropriate controller input icons, and automatically bring up the on-screen keyboard when needed.
The game should support the default resolution of Steam Deck (1280x800 or 1280x720), have good default settings, and text should be legible.
The title shouldn't display any compatibility warnings, and if there's a launcher it should be navigable with a controller.
If running through Proton, the game and all its middleware should be supported by Proton. This includes anti-cheat support.