@Override
    public void shutdown() {
        enableService( false, false );
        ClusterManager.removeListener(this);
        MUCEventDispatcher.removeListener(occupantManager);
    }
Determines if a stanza is sent by (on behalf of) an entity that MUC room believes to be an occupant
when they've left: a 'ghost' occupant

For message and presence stanzas, the delivery errors as defined in section 18.1.2 of XEP-0045 are used.

For IQ stanzas, these errors may be due to lack of client support rather than a vanished occupant. Therefore,
IQ stanzas will only return 'true' when they are an error response to an IQ Ping request sent by this server.

@param stanza The stanza to check
@return true if the stanza is a delivery related error response (from a 'ghost user'), otherwise false.
@see <a href="https://xmpp.org/extensions/xep-0045.html#impl-service-ghosts">XEP-0045, section 'ghost users'</a>
xmppserver/src/main/java/org/jivesoftware/openfire/muc/spi/MultiUserChatServiceImpl.java