https://github.com/NixOS/nixpkgs/blob/master/default.nix#L5 mentions https://nixos.org/wiki/How_to_update_when_Nix_is_too_old_to_evaluate_Nixpkgs, which points people to Hydra URLs that no longer work.

I remember a thread about improving that mechanism, but even if we don't do something fancy, we should have the URLs actually work. Now that the wiki is read-only, I'm not sure how best to fix it.