Valve has put up a Beta for Half-Life 2, Half-Life 2: Episode One and Half-Life 2: Episode Two as they prepare more of their own games ready for the Steam Deck.
We don't know yet all the exact details, as this update hasn't even been announced by Valve yet but the Betas are up and you can try them out right now. However, we do know for sure they now have DXVK Native which is the port of DXVK to Linux which allows it to be used natively without Wine. This changes the game to use Vulkan, instead of OpenGL, if you launch it with "-vulkan" in the launch options.
Other improvements noticed include some big bug fixes to a few scenes, UI improvements with resolution scaling with the HUD now having its own scaling option - which has resulted in the UI being a much better fit, ultrawide support, FOV now goes up to 110 instead of 90 and likely more we've missed.
To try it out all you need to do is opt into the Beta branch for the games on Steam. Right click -> Properties -> Betas (on the left) and then select it from the dropdown box. When they do put up full patch notes we'll let you know.
Hopefully at some point they will upgrade Team Fortress 2 and Counter-Strike: Global Offensive too.