alt Hacker NewsSo do Gentoo and Nix, yet they have packaging separate from the source code. The source is fetched, but the build is sandboxed from the network during the configure, build and install phases. So it's technically possible.
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Nix and specifically nixpkgs is IMO very bad at this. It's not a distro: it's a collection of random links that in many cases now only exists in cache.nixos.org. The tarball server frequently doesn't have content, can't represent some content at all (recursive hash types), links have rotted away completely (broadcom driver zips referencing a domain which is now advertising online gambling).
Nix isn't functional: it's a functional core that moved every bit of the imperative part to an even less parseable stage, labelled it "evaluation" and then ignored any sense of hygiene about it.
No: your dependency tree for packaging should absolutely not include an opaque binary from a cache server, or a link to a years old patch posted on someone else's bugzilla instance (frequently link rotted as well).
Nothing has made me appreciate the decisions of mainstream distributions more then dealing with an alternative like this.
Nix definitely does not allow most things to be built offline (at least in the way Debian means it).
With Nix, any fetcher will download the source. It does so in a way that guarantees the shasum of what is fetched is identical, and if you already have something in the nix store with that shasum, it won't have to fetch it.
However, with just a mirror of the debian source tree, you can build everything without hitting the internet. This is assuredly not true with just a mirror of nixpkgs.