Do we know what kernel SteamOS uses? Is it built on linux, or could it be some sort of kiosk'd mode Windows where this will be a non-issue? One could hope but I truly don't know.

SteamOS on the Deck is just a standard (tuned) Linux distribution under the hood. It would be very surprising to me if Valve shifted to an entirely different OS for the Cube.

It is running Valve's immutable fork of Archlinux, you can find their source package mirror online.