Comment by l0b0

It does, actually! Because only the package definitions are needed, and you can get those from the tarball of the relevant nixpkgs repo commit. And since building+installing is the same Nix command as downloading+installing, whether the binary caches still have the old package (or are even still up) is only the difference between a fast download and a slower build. This is different from most package managers, where trying to install anything old means spinning up a complex build environment and learning a bunch of new tools.

Of course, if even the original sources are no longer available, you'd have to change the broken URLs. And if the source has somehow been scrubbed from the Internet you're SOL