> The extreme end is nix, which actively requires you to have the upstream written with nix in mind because nix will basically demand you configure the source code in ways to accommodate for it.

I'm pretty sure this isn't true.

What does upstream emacs have to do for Nix to provide https://github.com/NixOS/nixpkgs/blob/nixos-22.11/pkgs/appli...

I will agree though that applications which have their own update mechanism or do other things that make reproducibility harder are much more difficult to create a Nix expression for.