From the MQTT version 5 specification:

MQTT is a Client Server publish/subscribe messaging transport protocol. It is light weight, open, simple, and designed to be easy to implement. These characteristics make it ideal for use in many situations, including constrained environments such as for communication in Machine to Machine (M2M) and Internet of Things (IoT) contexts where a small code footprint is required and/or network bandwidth is at a premium.

If you don't receive a commercial license from us (1byt3), you MUST assume that this software is distributed under the GNU Affero General Public License, either version 3 of the License, or (at your option) any later version. See the LICENSE.txt file for more information.

Commercial licenses are available, send us an email: customers at 1byt3.com

This is a pre-alpha release.

Nope, only version 5 is supported. There are no plans to support older versions of the MQTT protocol.

OASIS MQTT Version 5 specification