Minor 1. This section feels a bit repetitive. It tells me a little bit:
1387 A Server is NOT REQUIRED to support QoS 1 or QoS 2 PUBLISH packets. If this is the case it MUST
1388 include a Maximum QoS Advertisement in the CONNACK packet.
then it tells me a little bit more about Maximum QoS Advertisement after a pause:
1392 If a Server does not support QoS 1 or QoS 2 PUBLISH packets, it MUST send a Maximum QoS
1393 Advertisement containing the highest QoS it supports and MUST support all lower QoS Levels in addition
1394 to the value contained in the advertisement.
This could be compressed into one paragraph which would flow better.
2. It's the only case where "[Aa]dvertisement" is used.
3. There's a similar "flow problem below. I'd merge the Client and Server cases for a PUBLISH packet exceeding Maximum QoS together ...
1397 If a Client receives a Maximum QoS Advertisement from a Server, it MUST NOT send PUBLISH packets
1398 at a QoS level exceeding the Maximum QoS level specified.
1399
1400 If a Server receives a CONNECT message containing a Will QoS that exceeds its capabilities, it MUST
1401 reject the connection. It MAY send a CONNACK with a Return Code QoS-NOT-SUPPORTED and MUST
1402 close the transport connection.
1403
1404 If a Server has sent a Maximum QoS property, and it receives a QoS > 0 PUBLISH packet that exceeds
1405 the advertised maximum, this constitutes a Protocol Error and the Server must send a DISCONNECT
1406 packet with the Return Code QoS-NOT-SUPPORTED and disconnect the transport connection.
4. This section is a bit confusing because there's a CONNACK Maximum QoS property and a SUBSCRIBE Maximum QoS Subscription Option. It wouldn't hurt to be more specific.
1412 ... If
1413 this is the case, the Client simply restricts the maximum QoS field in any SUBSCRIBE commands it
1414 sends to a value it can support. A Client MUST NOT send a Maximum QoS Advertisement to the Server.