In RFC 7231 section 4.3.5 it says “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.”
I realize DELETE with a request body likely predates this guidance as it didn’t appear until well after the original HTTP 1.1 spec but this has become an issue for compliance with OAS 3.0.
Perhaps an alternate approach to deleting to-Many relationships might be an option for the next release of {json:api}? (Not that I can think of one that is as idempotent as the current approach.)