Since https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1038445 the json serialization format has not required the _complex_ property to be present, as it is redundant with the existing _type_ property.
After an appropriate interval (see https://www.mediawiki.org/wiki/Manual:Parser_cache/Serialization_compatibility#Cleaning_up_old_test_cases) we can remove the backward compatibility code in core's JsonCodec.