While the nix package manager supports the coexistence of multiple versions of a package, the nixpkgs package collection does not contain every single versions of every package. However, it does make it very easy to refer to past versions of a package by importing package specifications from an older version of nixpkgs.

I think this is a reasonable choice, considering that the main purpose of nixpkgs is to provide packages for the NixOS distribution. It's impossible to actively maintain every single version of every package.