Public review comment received from Nicholas Humfrey (point 2)

2) I was quite surprised to discover that an additional byte had been inserted in front of the return code byte in the Connack packet type, making it harder to maintain backwards compatibility. Given that the fixed header flags have now been made specific to the packet type, what was the reason for not using one of those? It seems like an expensive break in the existing protocol for a minor new feature? Particularly when there has been effort to preserve other less useful features of the protocol, such as setting flag bit 1 in the fixed header of a Subscribe packet.

Roger Light responded to this post, clarifying as follows:

The byte in question containing the "session present" flag just
replaces the already existing "reserved" byte of the CONNACK packet,
so there is no break in compatibility.