[pmahoney]

> if you want two versions, you need to ask the maintainers to split the package into two

This is still true in Nix (and Guix I assume), although there are facilities for overriding, say, just the source tarball (to a different version) without needing to create a whole new Nix package (and worst case, creating a whole new Nix package is an order of magnitude easier than creating a Debian package in my experience).

In the "nixpkgs" set of packages, a few packages come in multiple versions (gcc, autotools come to mind), but most are only available in a single version.

Of course, once you've managed to install package-version-1, even as nixpkgs moves to version 2, 3, and so on, your version 1 can remain installed forever if you wish, and nothing can swap out the libpng, or libc or whatever it may depend on, since as you say all these can live side-by-side with other versions of themselves.