Not only are Valve continuing to go over third-party games, they also continue upgrading their own with a fresh update out for Half-Life 2, Half-Life 2: Episode One and Half-Life 2: Episode Two.
All three of them recently had an update that noted "UI and Input support for Steam Deck verification", although only the original Half-Life 2 has been fully Steam Deck Verified so far. This is the finalization of work that went into a Beta recently, to give them a new gamepad friendly UI.
The original UI:
When set with "-gamepadui" as a launch option:
Considering how popular it still is, it's a good thing to have it ready for the Deck. A little Half-Life on the go? Will you be playing this one on Deck?