Nixpkgs' channel currently can't move forward so long as there is a
trace in evaluating the top-level arguments. Which means that it isn't
possible to add a warning message to warn users of future package
removal.

So the only way forward appears to be just removing the alias
altogether.

Things done

Built the manual and checked the release note did appear.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.