I sometimes write a flake with those 4 lines of Nix code, and it comes out just messy enough that tbh I’m happier adding an input to handle that. But I recently learned that the nixpkgs flake exports the lib.* helpers through nixpkgs.lib (as opposed to nixpkgs.legacyPackages.${system}.lib) so you can call helpers before specifying a system. And nixpkgs.lib.genAttrs is kinda close enough to flake-utils.lib.eachSystem that it might make a better solution.

Like where with flake-utils you would write,

flake-utils.lib.eachSystem [ "x86_64-linux" "aarch64-darwin" ] (system:
let
  pkgs = nixpkgs.legacyPackages.${system};
in
{
  devShells.default = pkgs.mkShell {
    nativeBuildInputs = with pkgs; [
      hello
    ];
  };
})

Instead you can use genAttrs,

let
  forAllSystems = nixpkgs.lib.genAttrs [ "x86_64-linux" "aarch64-darwin" ];
  pkgs = forAllSystems (system:
    nixpkgs.legacyPackages.${system}
  );
in
{
  devShells = forAllSystems (system: {
    default = pkgs.${system}.mkShell {
      nativeBuildInputs = with pkgs.${system}; [
        hello
      ];
    };
  });
}

It’s more verbose, but it makes the structure of outputs more transparent.