@kitten I guess technically you could add any game to Steam as a non steam game and launch it via Steam. That should work in any case. But I think it's likely that it's gonna have some drivers to default to and will just work anyways, even without steam.
@kitten I just noticed, the Steamdecks build-in controller works without Steam. Like, in desktop mode, if you shut off Steam it will be picked up by applications with controller support like heroic as a normal controller. (either as an xbox360 gamepad or generic something something. But I think it does x-input)
@kitten@mildsunrise the original steam controller has a special mode where it pretends to be a keyboard and mouse, and on some platforms (Android) that's the only mode that seems to work outside of special software (Steam Link App)
On desktop there's sc-controller, I think some games will recognize it without that but YMMV as far as the UX goes.
You can add any game to steam to use steam input with it fwiw, and then you get steam's configurator for the controller.