NEP-22 is the high-level overview of the goals, but NEP-18 is the concrete proposal for __array_function__. Quoting that NEP, right under “Implementation":

The __array_function__protocol, and its use on particular functions, is experimental. We plan to retain an interface that makes it possible to override NumPy functions, but the way to do so for particular functionscan and will changewith little warning. If such reduced backwards compatibility guarantees are not accepted to you, do not rely upon overrides of NumPy functions for non-NumPy arrays. See “Non-goals” below for more details.