I'm more interested in the Nix/Guix approach, where you get the benefit of exact version linking, but also an "immutable" version controlled OS, so things like Puppet aren't necessary to manage state. (And likewise, deploy small VMs/containers with just the package manager tools without needing Docker as the entire OS state can be rebuilt using a definition file/version hash.)

Yeah, it seems ideal. But I've not used it, yet, mostly because it's such a big change.