Where did anyone say "not recommended in prod"? The experimental tag is mostly saying they may change the API when they upgrade Nix itself.
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.
Stabilization of flakes and certain other experimental features is a top priority for the Nix team at the moment (disclaimer: myself included). Please take a look at RFC136 for an outline of our plan for the way ahead…… and please comment.
Have been using flakes in production for more than 2 years - I think it is stable enough for daily use.
Flakes seem to be in "beta" the way Gmail was for years. Just about everyone I know that uses nix uses flakes now, and I've mostly switched myself.