I guess that’s true, tbh the reproducibility aspect is really what I like about nix, and I guess I’m confusing a bit here. I guess I’m saying nix gives a good compromise with immutable generations and high repro, but you’ve convinced me it’s not immutable per-se.

If you control everything in the build it is, and every generation is immutable.

At the surface, you can pin the commit you pull packages from, but if you want to go deeper, you can essentially define your own channel and dependent binaries, allowing you to store every aspect of how a generation is built.

It can be made to be by pinning various things which are not by default.