let
  # niv sources
  sources = import ../nix/sources.nix;

  defaultArch = "x86_64-linux";
  defaultPkgs = sources."nixos-22.05";

  lib = import (defaultPkgs + "/lib");

  machines = {
    "foo.example.com" = {
    };
    "bar.example.com" = {
      packages = source."nixos-21.11";
    };
  };

  mkMachine = hostName: { system ? defaultArch, packages ? defaultPkgs, tags ? [] }:
    {
      imports = [
        (./machines + "/${hostName}")
      ];

      nixpkgs = {
        inherit system;
      };
    };
in
{
  meta = {
    nixpkgs = defaultPkgs;

    nodeNixpkgs = lib.mapAttrs
      (hostName: { packages? defaultPkgs, ... }: packages)
    machines;
  };
} // (lib.mapAttrs mkMachine machines)

{
  meta = {
    # Override to pin the Nixpkgs version (recommended). This option
    # accepts one of the following:
    # - A path to a Nixpkgs checkout
    # - The Nixpkgs lambda (e.g., import <nixpkgs>)
    # - An initialized Nixpkgs attribute set
    nixpkgs = <nixpkgs>;

error: attribute 'path' missing

       at /run/user/1000/.tmpsGzWiX:396:26:

          395|       else pkgs;
          396|     evalConfig = import (npkgs.path + "/nixos/lib/eval-config.nix");
             |                          ^
          397|     assertionModule = { config, ... }: {
(use '--show-trace' to show detailed location information)

let
  # niv sources
  sources = import ../nix/sources.nix;

  defaultArch = "x86_64-linux";
  defaultPkgs = sources."nixos-22.05";

  lib = import (defaultPkgs + "/lib");

  machines = {
    "foo.example.com" = {
      system = "aarch64-linux";
    };
    "bar.example.com" = {
      packages = source."nixos-21.11";
    };
  };

  mkMachine = hostName: { system ? defaultArch, packages ? defaultPkgs, tags ? [] }:
    {
      imports = [
        (./machines + "/${hostName}")
      ];

      nixpkgs = {
        inherit system;
      };
    };
in
{
  meta = {
    nixpkgs = defaultPkgs;

    nodeNixpkgs = lib.mapAttrs
      (hostName: { packages? defaultPkgs, ... }: packages)
    machines;
  };
} // (lib.mapAttrs mkMachine machines)

In reply to @blaggacao:matrix.org
In my opinion, maintaining legacy nix support in-tree is a bit of a stretch and reduces the ability to innovate. Maby there can be a 2-tree solution that is not 100% mutually compatible .