DELETE is like GET, it is to ask the server to delete a resource which identified by the uri.
In another word, the payload and content-type in Delete request has no meaning.
As per RFC 7231 - Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content ,
A payload within a DELETE request message has no defined semantics;
sending a payload body on a DELETE request might cause some existing
implementations to reject the request.
in an other word, payload(content-type) is allowed, but the server should ignore it, or reject it.
I done a test on gateway 9.3, it's true that the gateway will remove the content-type and payload when route the DELETE request to the backend -- gateway is also a server against the requestor, the gateway ignores the payload(content-type) for DELETE request is expected as per the RFC.
I am curious what's purpose the backend service expects content-type (ie. expects a payload) for a DELETE request?