Yes, BASIC's CallHTTP submitRequest function ignores the payload if the HTTP verb is DELETE. And this is for the reasons that Aaron mentioned in his answer - namely that the RFC for HTTP 1.1 has traditionally left this undefined. The latest RFC (https://datatracker.ietf.org/doc/html/rfc7231#section-4.3.5) states
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.
There is some discussion on this topic at Stack Overflow: https://stackoverflow.com/questions/299628/is-an-entity-body-allowed-for-an-http-delete-request