I'd be interested in seeing your encrypted push notifications spec being standardized. I think it's nice and straightforward, so thanks for publishing it!

The one thing I'm unsure about is the design decision to pick individual elements, such as the message body, groupchat nick, or Jingle negotiation stuff, rather than simply including the entire stanza. With the current solution, the spec and the server implementations would have to be extended whenever some client needs another field for some reason, like you already had to do for Jingle Message Initiation and OMEMO.

I guess the motivation of your approach was minimizing the amount of data, as both APNS and FCM limit the payload size to 4 kB. However, I'd assume that hitting such a limit in practice will usually happen due to a rather long message text, rather than huge amounts of additional XML elements, so the win in omitting those will usually not be all that significant. And the spec could still allow (or explicitly suggest) the server to remove uninteresting elements and/or truncate unencrypted <body/>s.

Either way, if payload size does end up being a problem, another option might be compressing the data?