Nix Flakes are deemed experimental and explicitly not recommended in prod by the Nix project, afaict. Despite these warnings it’s widely used, but breakage is very much a possibility because it hasn’t gone through the Nix RFC process.
Also, "in prod" is such an odd thing to say about Nix. It's a built-time thing. Nix doesn't run in prod. Even NixOS isn't running Nix code. It's running systemd units.
What people mean by "use X in production" is that X is a key dependency as part of the production process.
If you describe a NixOS configuration for the system where your services are running, you're "using" Nix, even though the Nix-specific stuff is already done.
This relates to risk. -- If it turned out that using Nix to build packages or to configure a system then caused problems when running services later, that would be a problem. Whereas, "using in prod" suggests confidence in no problems, or in being able to fix any issue in a timely manner.
First, the non-Flakes CLI wll be stabilized, in phases.
Afterwards, Flakes itself and its CLI components can be stabilized. The final design of Flakes will also require another RFC.