What are the defaults for policies in Core?

What are the default featurizers for each policy?

Or, is Maxhistory also used by Keras Policy?

What are the recommended featurizers?

The default ones are the recommended ones. This is the standard featurizer for any policy (unless overriden): MaxHistoryTrackerFeaturizer(BinarySingleStateFeaturizer()). So this is what the KerasPolicy uses.