We recently pushed an early preview of history sync in the Communiqué TUI client. History syncing in XMPP is accomplished with a deceptively simple query system called Message Archive Management (MAM) that makes it easy to get history for a chat or two, but fiendishly difficult to do in such a way that's fast and efficient for large chat systems that need to be bootstrapped on every startup. This is likely part of the reason (alongside NIH syndrome) that commercial XMPP clients like Google Talk and HipChat never adopted MAM.

We've outlined our strategy for sync in the ModernXMPP docs in the hopes that it will be useful to others. We'd love to see other clients start outlining their own MAM strategies here as well so that we can all benefit from the shared knowledge!

https://docs.modernxmpp.org/client/sync/