Truth testing states that "Any object can be tested for truth value" but from 3.9 onwards, doing so with NotImplemented is deprecated and will be made a TypeError.

https://docs.python.org/3/library/stdtypes.html#truth-value-testing

It is also not true for third-party objects such as numpy arrays (which raise ValueError) and pandas dataframes.

I think that truth testing should have been considered a fundamental operation that (in the absence of bugs) always succeeds, but #35712 says different. Not that I'm bitter *wink*

In any case, at the very least the exception for NotImplemented should be documented.