You must use the plasma-mobile stuff, not the plasma desktop.
Mobile wasn't packaged for NixOS.
I am curious why/if Valve hasn't used it for SteamOS's desktop mode though.
Package it yourself then!
I don't really know, but I don't think the use outside of steam is an important use case to them, possibly they don't want to make things too easy, or maybe it wasn't packaged for arch either when they created the base image.
Mobile wasn't packaged for NixOS.
I am curious why/if Valve hasn't used it for SteamOS's desktop mode though.
Package it yourself then!
I don't really know, but I don't think the use outside of steam is an important use case to them, possibly they don't want to make things too easy, or maybe it wasn't packaged for arch either when they created the base image.