I plan to do a reproduction of the 25.11 minimal iso, avoiding the binary cache as much as possible - but https://github.com/NixOS/nixpkgs/pull/468139 means it'll likely take a 25.11 staging cycle before I can...
https://blog.josefsson.org/2025/12/07/reproducible-guix-container-images/ was interesting. It's not quite clear to me whether they mean 'from source' reproducibility or that they rely on their binary cache while building the container images, though?