This would solve a major issue with SteamOS. The SteamOS itself is immutable/read only as you know. This means, one cannot install or make changes to the core system (it is possible, but that is temporarily until update will revert those changes back). That is why we have the Flatpak support. But many software are not available there (especially CLI tools, if you use them).
This is where the Nix package system comes into play. In short, it allows for installation of programs without touching the immutable part of SteamOS in a controlled and safely manner. These would persist even on SteamOS system updates. For a better explanation, have a look here: https://nixos.org/guides/how-nix-works.html
Valve confirmed: https://github.com/NixOS/nix/issues/7173#issuecomment-1603858003 that Nix package support will be integrated into SteamOS! That’s huge. In example I have tools which I cannot distribute easily for Steam Deck users, because Flatpak does not support CLI apps. Nix would solve this issue!
This is probably not something I will make use of at all in the near future but I still absolutely love that they are doing this! Valve is really carving out a path forward for linux as a whole and eventually normal folks - not just enthusiasts - will be able to drop Windows completely.
If Valve really allows nix then future mods & stuff can be provided through that. As long as a person manages to create a nix config for whatever everyone can let nix replicate that on any device anywhere and anytime.
That guy managed to install and run Blizzard games? Share the config.
That other guy is able to tweak an intricate feature? Just share the config.
How much knowledge do gamers need to apply that config? Close to nothing.
It’s a really neat thing.