For flake.nix, you add that to the inputs, for example, call it `nixpkgs-old`, and then reference the package of interest with:

`nixpkgs-old.legacyPacakges.linux-x86_64.node`, or something like below

  let pkgs = import nixpkgs-old {
    system = "linux-x86_64";
  } in pkgs.mkShell {
    packages = [ pkgs.node ];
  }