The default makes the stored value JPA-compliant by storing the last generated value. The old behavior was to store the generated value to be used next.

If this were to be backported to 5.2 branch, then the default for that branch should be false to maintain the old behavior. If an application explicitly set hibernate.id.generator.stored_last_used=true, then later used the default, then Hibernate would try to reuse the same value.

I don't think backporting is worth the complication of a changing default going from 5.2 to 5.3, and possible failures due to changing the value for the property in existing applications..