The GTK+ 2 version of Claws Mail, major version number three, relies on Python 2, which is end-of-life and might be dropped in the nixpkgs. In favour of #148779, this older branch of Claws Mail was removed.
Moved to ./contributing-to-this-manual.chapter.md. Link:
https://nixos.org/manual/nixos/unstable/#chap-contributing