At the #XMPP Summit − after a topic is well-discussed and a rough consensus reached − someone needs to volunteer to write the XEP.
Last year, I volunteered to write the XEP for synchronizing read states across devices.
It's called Message Displayed Synchronization and is now in Last Call, where community feedback is gathered before a XEP becomes 'stable.'
https://xmpp.org/extensions/xep-0490.html
Prototype implementations are in @Monal and #Conversations_im.