The Entity serializer should alow meta-data like revision ID and timestamp to be injected into the serialization data. This should be done in a consistent manner where appropriate. In particular, this should be used in the output of Special:EntityData.
This blocks "WikibaseDataModelSerialization with what we do with WikibaseLib" and although we have totally removed the Lib Serialization I would not consider this task complete in regards to WikibaseDataModelSerialization..
As far as I'm concerned this is not something that should happen in DataModelSerialization at all. Not sure what to change the issues title to though. "Maybe improve the code adding meta-data to the API output somehow" is a bit vague, yet seems to be all that is left.