My life before flake-utils.lib.eachDefaultSystem was:

before-flake.nix

...
let
      lib = nixpkgs.lib;
      systems = [
        "aarch64-linux"
        "x86_64-linux"
        "aarch64-darwin"
        "x86_64-darwin"
      ];

      forAllSystems = nixpkgs.lib.genAttrs systems;
      forEachSystem = f: lib.genAttrs systems (system: f pkgsFor.${system});

      pkgsFor = lib.genAttrs systems (system:
        import nixpkgs {
          inherit system;
          config.allowUnfree = true;
        });

      devShellFor = system:
        let
          pkgs = import nixpkgs {
            inherit system;
            config.allowUnfree = true;
          };
          script = import ./shell.nix { inherit pkgs; };
        in
        script;
      pkgs = nixpkgs.legacyPackages.${system};
    in
...