Is this the RFC you refer to? [RFC 0136] A plan to stabilize the new CLI and Flakes incrementally by Ericson2314 · Pull Request #136 · NixOS/rfcs · GitHub
And maybe this is the tracking of it? CLI stabilization effort · Issue #7701 · NixOS/nix · GitHub
Maybe that’s a thread on it Stabilising the new `nix` command line interface ?