To enable the use of flakes, you have to use the 'extra-expiremental-features flakes' flag.
Edit: Apparently they are called 'extra-expiremental-features' not 'extra-unstable-features'. Regardless the nix docs explicitly describe them as unstable here
They're "experimental" because the RFC wadn't approved IIRC, but everyone in the community (and the new documentation) talks about amd used the new commands.
Flakes aren't really unstable, but a lot of documentation is based on the old way of doing things. Also, Nix(OS) has a steep learning curve.
To enable the use of flakes, you have to use the 'extra-expiremental-features flakes' flag.
Edit: Apparently they are called 'extra-expiremental-features' not 'extra-unstable-features'. Regardless the nix docs explicitly describe them as unstable here
https://nixos.org/manual/nix/unstable/contributing/experimental-features.html
They're "experimental" because the RFC wadn't approved IIRC, but everyone in the community (and the new documentation) talks about amd used the new commands.