Sessions are obsolete and were removed from the XMPP spec in RFC 6121.
Sessions in the old RFC: http://xmpp.org/rfcs/rfc3921.html#session
PR submitted: https://github.com/igniterealtime/Openfire/pull/146
We can mark the xmpp-sessions capability as "optional" to support older clients that still expect (require) it. Refer to https://tools.ietf.org/html/draft-cridland-xmpp-session-01 for details.