It's unfortunate that technologists continue to propagate serious mistakes like "[...] its also clear that REST and its inherent polling mechanism isn't the best way of building a user notification system [...]"
REST is about state transfer - and event notifications are also state transfer.
As for HTTP, it isn't only "polling" - anyone that has posted a blog entry knows that. The 'client' can 'post' updates to the 'server' - exactly the same as event notifications via XMPP. The great thing about XMPP is the federated multi-hop capability with 'trust' built-in. Just like email, only with everyone using settings for very low latency delivery.
There have been multiple publish/subscribe over HTTP mechanism (comet, mod_pubsub, KnowNow, etc) over the years.