It’s less about stability (things rarely break, which is great). More about frequency of updates. Whenever I nix-channel --update on unstable I need to do a lot of re-downloading (a bit annoying with slow internet), and a whole lot of rebuilding for customisations and other package sets which aren’t in nixpkgs (e.g. lots of ocaml). I assume (untested) there would be fewer rebuilds required on a stable branch. Pretty much everything would need a rebuild every 6 months when the next release came out, but outside that there should be a slower rate of change.
For clarity I wasn’t suggesting this would replace the actual stable release branches, just a meta-branch tracking the “latest” stable branch. On a production system I’d pick a specific release, but for my home system it’d just be like “unstable” but on a larger timescale, with far fewer incremental updates required between release versions.
There may be a better name than “stable” though. “nixos-release”?