If your NixOS setup is not of hello-world kind, you typically have sops in it (or another secret management tool) and want to backup your age keys as well. They're necessarily separated from the main config tree so they don't end up leaking to the globally accessible nix store, and in the most typical case they're located in a 600 file outside the git repo. Forgetting to back it up is a classic NixOS gotcha and also a generalized version of this koan.