I fear that, just like the Steam Deck's controller, it won't be usable without Steam running. IMO by default and without any special "driver" running in the background, the sticks and buttons should just behave like a Xbox controller.
The original Steam controller worked without Steam running, even including some of the extra features like mouse and scrolling functions for the trackpads if you wanted it to. So here's hoping
The Steam Deck's controller is usable without Steam running, except for that long, looong pause when Steam has taken over the controller but isn't doing anything about it yet.
I wish that Steam would not put the mouse buttons on the triggers, just leave them on the trackpad-click. And put "high res trackpad scrolling" on the left pad. But you can't have everything.