Undefined terms in RDFS namespace

This is a late comment on RDF Vocabulary; I will understand if it is not
addressed.

For undefined terms in the RDF namespace, the syntax spec says:
http://www.w3.org/TR/rdf-syntax-grammar/#section-Namespace
[Any other names are not defined and SHOULD generate a warning when
encountered, but should otherwise behave normally.]

There is no such indication of appropriate behaviour with rdfs:foobar.

Is this deliberate, or an omission?

Jeremy