`nix-env` can be coerced into pinning a package at a specific revision, effectively ignoring channel updates. However, yes, I do all of my nixos package management in my system configuration and try to avoid `nix-env` at all costs as it leads to drift. However, in at least one case, this is largely un-avoidable (in the case of OBS Plugins, there's no NixOS infra to make sure that 'wlrobs' is available at a well-known spot for OBS to pick-up. We'd need to make a plugin-aware wrapper, or `programs.obs` module probably. This is because nix-env does some additional symlinking that isn't done for system-installed packages).