For most people, depending on historic versions of nixpkgs is an anti-pattern. The main reason I say this is security. You may want an old version of, say, ruby, but you probably don't want the old, insecure libraries it was built against (i.e. glibc, openssl).

It is often just as easy to use overrides to build a different version of a piece of software, and in the rare case that doesn't work you can fall back to a custom derivation (drawing inspiration from the old version of nixpkgs, perhaps).

I worry about tools like this which make doing the wrong thing too easy.