As far as I can tell nix has no log. I'm guessing the reason for excluding this feature of every-other-package-manager-ever is that nixos is declarative rather than imperative, so the process it went through to get your system from one state to another should be irrelevant.

Which brings us to what I suspect you really want to know: How can I compare the current generation with the last?

I've found nixos-scripts' diff-generations command very helpful for this. For instance, if you're on generation 115, you can get a diff of installed packages with: