If you want to install an old version that has been packaged in the past, you can just find the Nixpkgs commit with the desired version and use that (you could use nix-versions or nixhub to search past versions). So there’s no need to keep old versions around since you can always go back.
If you want a specific version that was not packaged, you probably will need to do the packaging yourself. It’s not the goal of Nixpkgs to have every single version of every package available that there ever was.