Flakes are neither stable or unstable...

They are defacto stable in that everyone is using them and the interface is not really going to change at this point.

But they are still marked as an experimental feature.

To get them out of experimental, the requirements are

normalizing the CLI. Currently some options are global and some are local to a command. The plan as I understand it is to make them all local. This would make it clear which options any particular command actually take instead of guess work. They are not going to break usage, but make the usage more clear.

And fetchTree needs to be stabilized. A big criticism of flakes is that they had a nonstandard method of fetching data and repos. fetchTree is a new nix built-in that behaves the same way. This makes flakes a lot less special.

So, nix flakes are a stable experimental feature, slowly getting to the state that it won't be experimental any more.