Previous version was in bash. With this change you can build a nixos image not containing bash or any shell whatsoever.
Not having interpreted languages on the system at all is an effective hardening technique combined with verity store containing all your executables as it makes it impossible for attackers to add new executable files to the system which stops almost all attack vectors.
You can read about the project here: https://github.com/NixOS/nixpkgs/issues/428908