Denormalization on field items is now called

When denormalizing fieldable entities previously, all entity values were passed to the entity create() method and the object was constructed via the entity system alone. The serialization module now supports field level denormalization, similar to the hal module. So custom denormalization behaviour at the field level is now possible, with each set of field values being passed along.

See \Drupal\serialization\Normalizer\FieldNormalizer::denormalize() and \Drupal\serialization\Normalizer\FieldItemNormalizer::denormalize() for examples of this in the core serialization module.