it seems the actual scope of the exam does not contain firestore in the exam guide but does have a mention of cloud data store.. https://cloud.google.com/certification/guides/data-engineer.
would we see questions on both in the exam?
Cloud Firestore has been the preferred solution for a while now but it does seem to be taking a while for the exam curriculum to catch up. Most of what you learn about Firestore applies to Datastore (they are both the same sort of database) but you should probably be familiar with the major differences just in case Datastore pops up on the exam. Overall, Firestore has many improvements, the main one being strong consistency. Firestore is not supported by the (rather old) App Engine Standard runtimes for Python 2.7 and PHP 5.5