With we simplified the fields needed to encode IdSourceKey, which certain dialects serialize directly into a buffer stored by the data store (e.g. Ehcache and Infinispan embedded).
We decided to not break serialization compatibility, however we should introduce a flag or a strategy to eventually move on to the leaner format.
Especially new applications should be encouraged to use the new schema.
See Ehcache's SerializableIdSourceKey and Infinispan's IdSourceKeyExternalizer