XEP-0184: Message Delivery Receipts
"While Advanced Message Processing  provides message acknowledgements at the server level, it does not extend that model all the way to the client.  However, sometimes client-level acknowledgements are needed, for example to provide "receipts". This document defines a mechanism for XMPP message delivery receipts, which are functionally equivalent to the "delivered" or "displayed" event in Message Events , which this specification in part obsoletes."
Perhaps after 3.1.0 release this feature is moved to an extension
Available in the Message class