"a. The following reports should be made to
ATC or FSS facilities without a specific ATC
1. At all times.
(a) When vacating any previously assigned
altitude or flight level"
In FAA orders "should" is not the same as "shall". Shall indicates a requirement. Should is not a requirement, however it is highly recommend.
Incidentally the AIM itself isn't an order nor is it regulatory. It's informational, but it is
based on regulations. That's a whole other discussion however.
d. This publication, while not regulatory, provides
information which reflects examples of operating
techniques and procedures which may be requirements
in other federal publications or regulations. It
is made available solely to assist pilots in executing
their responsibilities required by other publications.