How to ensure that NixOS configuration will build or use the same package versions in the future?

In Software packaging and distribution for LHCb using Nix , the authors write: > In order to facilitate this use, software must be stable for long > periods; much longer than even Long Term Support operating systems are > available. Additionally, the software should reproduce any and all > bugs which were present in the original version to ensure the accuracy > of the final results. Builds should be reproducible to allow for > patches to be carefully introduced. But NixOS configuration files do not include versions of packages (unlike, say, Rust manifests), e.g.

environment.systemPackages = with pkgs; [
    git
    git-lfs
    fish
    neovim
    nixpkgs-fmt
    nixos-option

    # Basic utils
    killall
  ];

If I understand correctly, packages can be updated within a channel and they can change when channel is changed. Then, how to ensure that in 10 years I will be able to get or build the same Nix environment with the same versions packages installed?