XEP-0085: Chat State Notifications is a client-side protocol. However, it mentions one duty on the server-side:
a server or service MAY refuse to deliver standalone notifications to its users, and SHOULD NOT store them offline.
It also explains:
A "standalone notification" – that is, a message stanza that does not contain standard messaging content but instead is intended to specify only the chat state since it contains only a child element qualified by the "http://jabber.org/protocol/chatstates" namespace (or possibly also the XMPP <thread/> element; see the Threads section below).