wesnel
I would imagine that you could use Home Manager on a non-NixOS system to at least create reproducible configs for the programs you use, although the OS as a whole would of course still be non-reproducible. However, I do not know how well Home Manager works on non-NixOS systems.
As you mention, just using Nix itself can be sufficient to get a reproducible set of packages on your system. I recall reading a blog post about someone who does this on both Ubuntu and MacOS.[1] The way this person does it is interesting because it’s more sophisticated than spawning the occasional ‘nix-shell’ or something. For example, they get the benefit of Nix “generations,” with a new generation being created each time they modify their declarative config files.
[0] https://github.com/nix-community/home-manager [1] https://www.nmattia.com/posts/2018-03-21-nix-reproducible-se...