When you want to reference a package from another package, or a devShell, etc... inside eachSystem/eachDefaultSystem closure then you have to either use the pesky rec keyword which even does not work for all cases or you have to use self parameter from the flake itself like self.${system}.packages.abc which is unnecessarily repeating the system.

I propose adding a variant of eachSystem/eachDefaultSystem which could be used like this:

{
  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem2 (sself: system: {
      packages.abc = ...;

      devShells.default = pkgs.mkShell {
        packages = [
          sself.packages.abc # Instead of self.${system}.packages.abc;
        ];
      };
    });
}

Perhaps it could be made backward compatible similar to overrideAttrs (NixOS/nixpkgs#119942) so new variant of eachSystem may not be necessary.