The specification currently states:

The DataTypeDefinition Attribute is used to provide the meta data and encoding information for custom DataTypes.
It is then followed by requirements to provide this attribute for Structure and Union DataTypes as well as Enumerations and OptionSet DataTypes. The question from a certification has now been raised how such types defined in companion specifications fit into the game. Are they considered a custom datatype (since not in the OPC UA core namespaces) and mandated to have the DataTypeDefinition attribute or are they still standard types (since they are not custom) and therefore not mandated to have the attribute?

After discussion and thinking about the different use cases I would even say, that any of these Types need to have the DataTypeDefinition Attribute, even the ones defined in future OPC UA Core spec versions. Since OPC UA Clients will use this attribute to learn the datatypes there would not be interoperability between a server based on a newer specification with new structures that an older client doesn't know about but has a generic structure handling. Note, this just looks like a spec issue since the most common demo servers have the DataTypeDefinitions set also for all namespace 0 DataTypes.