Packages that make it into nixpkgs (and are not ignored by Hydra because e.g. they're non-free) get copies of their fetched sources included in the nixpkgs bina

Packages that make it into nixpkgs (and are not ignored by Hydra because e.g. they're non-free) get copies of their fetched sources included in the nixpkgs binary cache. So they're there for "posterity" (there's no such thing as posterity, though I'm sure some wise people are maintaining full personal backups of the nixpkgs binary cache).