Too bad you couldn’t do something open source like that since it requires the D&D licensing.
You don't have to use any d&d system or content to make something similar (tho tbh I don't think it's a good system for videogames anyway), but especially on the content side it's hard to find a team dedicated enough and founds enough to make it happen
I don't know what you mean by "steamdeck version", but afaik the SteamOS you can download from steam is version 2 which is really old by now while the Steamdeck runs version 3 and is still not available for download outside of it. Mind you it isn't just a major version change but they even changed the linux OS it's based on, moving from Debian to Arch.
As for the replacement for desktop, the Steamdeck (and with it SteamOS 3) launched with full desktop mode available and while the core system is immutable as discussed, it's still a fully functional desktop OS and as such it depends on your personal needs how much of a windows replacement it can be, just like any other linux OS. Also you might need a dock or other USB hub to attach a keyboard and mouse if you need them, but there is onscreen keyboard and touchpad on the steamdeck available.
To my understanding it's one step more, not only observation but interaction as well, manipulating one side of the entangled particles resulting in an instant (and similar/same) change on the other side as well. In fact even the observation is a form of interaction in this case that changes the state of the entangled system.