All statements in responses should include the data type of the of the Property in the mainsnak.datatype field of the statement. Note that this is different from the datavalue.type field. See https://gerrit.wikimedia.org/g/mediawiki/extensions/Wikibase/+/9b6858649d8333e664500f1635a2205f6daf28d0/docs/topics/datatypes.md
The datatype field should be required on the mainsnak field of a statement in the *response schema* . The datatype is not expected in the request data.