The existing text OData Version 4.0 Part 1: Protocol (oasis-open.org) says the following:

11.4.5 Delete an Entity

To delete an individual entity, the client makes a DELETE request to a URL that identifies the entity. Services MAY restrict deletes only to requests addressing the edit URL of the entity.

The request body SHOULD be empty. Singleton entities can be deleted if they are nullable. Services supporting this SHOULD advertise it by annotating the singleton with the term Capabilities.DeleteRestrictions (nested property Deletable with value true) defined in [OData-VocCap].

>>The highlighted text applies to top-level <Singleton> entities, not to single-valued navigation properties. Calling DELETE on the edit URL of a nullable single-valued navigation property sets that property to null.  Subsequent GET requests would return 404.