You may send POST /events instead. It also breaks “REST”, which is just a sort of obsession rather than a requirement here, but more importantly it wouldn’t break idempotence and proxy caching that GET implies.

Edit: from the network point of view, it’s either call-back or a persistent call-wait/socket, or polling. The exact protocol is irrelevant, because it’s networking limits and efficiency that prevent everyone from having a persistent connection to everyone. A persistent connection can’t be much better than any other persistent connection in that regard, and what happens inside is unrelated story. Or am I missing something?