The schema provided by XEP-50 reads
<xs:elementname='note'>
<xs:complexType>
<xs:simpleContent>
<xs:extensionbase='xs:string'>
<xs:attributename='type'use='required'>
while section 4.3 states
The "type" attribute specifies the severity of the note. This attribute is OPTIONAL, and implies "info" if not present. The value of this attribute MUST be one of the following:
I consider the schema to be more authoritative. It's also what's easier to implement. But will ask on the XMPP standards list for clarification.
Thanks for pointing this out.