RFC 7540 also defines an optional way for the client to signal whether it wishes the request to be processed asynchronously: https://tools.ietf.org/html/rfc7240#section-4.1
To put it differently, why would it matter if a client POSTs with Prefer: respond-async if the server response will always be async? Whether that header is present or not, the HTTP response is already designed to always return a Location header.