[Filligree]

When was that added? Well fetchurl automatically poke it if it can't find the sources otherwise?

[ymse]

Yes, fetchurl will try it:

https://github.com/NixOS/nixpkgs/blob/733151b7825ee8840f7437...

Edit: It was apparently added in 2007:

https://nixos.org/releases/nixpkgs/nixpkgs-0.11/release-note...