Minor Line 1373-1374:
"it is advised that the Server does not to send a CONNACK at all, ..."
--> "the Server may choose to not send a CONNACK at all,..."
The non-normative comment talks about processing sensitive data, which may or may not be related to DOS attacks. Even if the Server doesn't process sensitive data, DOS attack may have high business impact.
Advising what to do is too strong statement, w/o the specific context. Saying, a Server could do it if there is such a concerns is more appropriate, and explains enough why a CONNACK is an optional packet.
This issue was originally reported in MQTT-417, which doesn't seem to be applied in WD14.