The Session Expiry Interval Property (0x11) can only be present in the CONNECT and DISCONNECT header.
For clients connecting to a MQTT broker it may be a useful information how long the session expiry interval is, especially if an administrative session expiry interval was configured on the broker and the client is not aware of it.
Also, if a client sets a session expiry interval in the CONNECT message, it may be overridden by the broker implementation. Currently there is no way to notify the client of his "new" session expiry interval.
So the proposal is to add the Session Expiry Interval Property as valid property for the CONACK packet.