The reason is probably moving to Netlify for nixos.org.
The links Repology uses to update packages are as follows:
nix_stable
: https://nixos.org/nixpkgs/packages-nixos-19.09.json.gz
nix_unstable
: https://nixos.org/nixpkgs/packages-nixos-unstable.json.gz
What are the correct URLs? I can open a PR to repology-updater
and fix them.