[Boulth]

> Just curious, which XEP is offline messages?

Simple retaining messages until the client is online was supported since ages, I don't think there is XEP for that but I could be wrong. More elaborate scheme with persistence, multidevice access, paging etc is here: https://xmpp.org/extensions/xep-0313.html

> In Matrix, clients are supposed to implement the full Client-Server API. If a client leaves out e2e then it cannot claim to implement the matrix protocol.

Yes, XMPP has XEP suites that serve the same purpose: https://xmpp.org/extensions/xep-0387.html

[monort]

Just a note - without XEP-0198 you will be losing offline messages if your connection is unreliable. I think most desktop jabber clients still do not support it.