https://search.nixos.org/packages only shows packages that are available on the latest stable and unstable releases of Nixpkgs. The means they only have a limited number of versions for a given package available. For example, Python on unstable only has 5 versions to choose from:

  - 2.7.18
  - 3.9.17
  - 3.8.17 
  - 3.11.4
  - 3.12.0b3

If you need something different, you need to find a specific commit of the NixOS/nixpkgs repo that has that version. This information is not indexed or searchable on the official package search. Nixhub.io provides an index of these older released commits, so you can find and install versions that aren't available in the latest Nix releases.

If you visit https://www.nixhub.io/packages/python, you can see we have a lot more versions available. Developers can use the Nix or Devbox references on that page to install the version they need.