Though the text and examples for the Element json representation have been clarified between DSTU 2 and now to remove the _id mention from examples and descriptions and make it consistent, there's still a mention of _id:

_The name of the property comes from the context in which it is used, not from the type. The internal id is represented as a JSON string property named "_id". _

All examples are now correct and just list an id property, this mention seems to be the last remaining vestige of the prefixed id that remains. This has caused confusion from others using our implementation, as they expect the Element.id to be Element._id instead.