The MessageEvent code system is empty.

http://build.fhir.org/codesystem-message-events.html

There is no indication why. It would help to say that it is deliberately empty and can be extended per application. Otherwise it just looks like a publishing error - especially since it was there in STU3. One of the students on the HL7 FHIR Fundamentals course was confused by this.

Also, this is normative, which doesn't really make sense. I don't actually object to that - if it will never have content, then that sort of works. But it needs the situation to be clarified with text.