"Should" is a term meaning that it's recommended, but not mandatory. Because we don't require the property to be present, we've been asked to provide guidance for clients for what to do when the property is not present.
If the property were mandatory, you would see "shall" instead of "should".
As Mike points out, the words "should" and "shall" have specific meanings in the specification, as explained in the "Terms, definitions, symbols and abbreviated terms" clause near the front of the document:
The terms "shall" ("required"), "shall not", "should" ("recommended"), "should not" ("not recommended"), "may", "need not" ("not required"), "can" and "cannot" in this document are to be interpreted as described in ISO/IEC Directives, Part 2, Clause 7.
As the DMTF contributes the Redfish Specification to ISO, we follow their formal language throughout the specification and the "long description" text in the schema files. And yes, you can read any sentence that contains the word "shall" to mean "this is required functionality", and anything else is optional, but recommended - otherwise we wouldn't have put it in the specification.