This post lists the IoT protocols supported by Amazon AWS' IoT, Microsoft Azure's IoT Hub and Google Cloud's IoT Core as of May 28th 2019.
Amazon AWS' IoT supports [link]:
MQTT
MQTT over WebSockets
HTTPS
Microsoft Azure's IoT Central supports [link]:
MQTT
MQTT over WebSockets
AMQP
AMQP over WebSockets
HTTPS
Google Cloud's IoT Core supports [link]:
MQTT
HTTP 1.1
References