The following informal mini-conversation occurs regularly in English:
I'd like to talk to you.
-- About what?
Any grammatical rule that would simply preclude the use of "About what" is a prescriptive rule. A descriptive rule would say, "The idiomatic way to tell someone, rather impatiently and brusquely, that they are not making themselves clear, is to ask What are you talking about?"
About what are you speaking? is "incorrect" insofar as it departs from idiomatic usage. Do you understand how that could be considered a far greater infraction than a violation of some prescriptive rule that has no basis in the spoken tongue?