My first impression is that RFC7540 isn't appropriate for this use case, because it is proposed as a way for the client to signal optional/prefered behavior when said behavior is already made mandatory by the server.
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.
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.