My last understanding is that Flakes are slated to go into stable in the next NixOS version 21.11 this fall. Do you know if that's still the case? I'm still just using configuration.nix for everything, haven't migrated to Flakes yet.

No. Flakes will be stabilized with nix version 3.0, while the current nix-unstable release is 2.4pre-something. See the roadmap here: https://discourse.nixos.org/t/nix-release-schedule-and-roadm...

I don't love that a post from July talked about stabilizing and releasing 2.4 "in the next few weeks", and now it's September with still no release. OTOH, the list of milestone issues is pretty short:

https://github.com/NixOS/nix/issues?q=is%3Aopen+is%3Aissue+m...

I expect the delay is mostly due to all this debate raging about `nix shell` naming and semantics: https://github.com/NixOS/nix/issues/4715

Thanks!

The main issue for me is that I'm working on packaging for a proprietary project where the ability to have an overlay flake and lock all my dependencies is critical. So I'm doing this with a flake and it works great, but I really didn't want to have to learn the old-and-busted way of doing it only to have to migrate later, particularly when the ergonomics of flakes is so great.