They shouldn't, though, because pretty much every XMPP server I've seen has user-offline message storage for later delivery eg [1], [2], and the "best practices" have been documented for years, e.g. [3].

Unless you actually mean "receive messages [to your device] when you're offline" and how the hell would that work?

[1] https://prosody.im/doc/modules/mod_offline [2] http://www.process-one.net/docs/ejabberd/guide_en.html#htoc4... [3] http://xmpp.org/extensions/xep-0160.html