From http://hl7.org/fhir/2018May/http.html#3.1.0.5.2

In the absence of the header, servers may choose whether to return the full resource or not (but not the OperationOutcome; that should only be returned if explicitly requested). Note that this setting only applies to successful interactions. In case of failure, servers SHOULD always return a body that contains an OperationOutcome resource.

Does that mean that informational and warning OperationOutcomes cannot be returned? This seems overly restrictive.