> As an example, nixos keeps state around regarding user id/username mappings, to avoid giving the same user id to different users across time. So a fresh install of nixos might leave services unable to read their data files, because the file might be owned by a different user id.

One reason to set `mutableUsers = false`: https://mynixos.com/nixpkgs/option/users.mutableUsers.

> And if you activate and enable incus, for instance, it will probably create a bridge device: the device will remain in place after you remove incus, which will have implications for how your network/firewall works that your configuration will depend on but will not enforce or be able to reproduce.

Impermanence: https://github.com/nix-community/impermanence.

To be clear, I don't use neither. But you can get NixOS to be almost completely stateless (if this is something you care) with a few changes. The power is there, but it is disabled by default because it is not the pragmatic choice in most cases.