Since nobody's brought it up: MQTT.
It got pigeonholed into IoT world, but it's a pretty decent event pubsub system. It has lots lf security/encryption options, plus a websocket layer, so you can use it anywhere from devices, to mobile, to web.
As of late last year, RabbitMQ started suporting it as a supported server add-on, so it's easy to use it to create scalable, event-based systems, including for multiuser games.