See

So, PyNumber_InPlacePower just ignores the o3 now. Maybe its docs also should be adjusted?

Look at wrap_ternaryfunc(). The __ipow__ wrapper accepts 1 or 2 positional arguments (not including self). An extension type can implement slot_nb_inplace_power that does not ignore the third argument.

An extension type can implement slot_nb_inplace_power that does not ignore the third argument.

How you can invoke it? (excluding a direct call of the dunder)

By a direct call of __ipow__().

"excluding a direct call of the dunder"?)

Don't exclude it. Signature describes the direct call.

Well, why someone will prefer a dunder method over a public method, that does same?)

Edit: similar to #122260 (3-arg __rpow__() - also expected to be directly called).