From https://github.com/hibernate/hibernate-orm/blob/5.0/migration-guide.adoc#type-handling

Built-in org.hibernate.type.descriptor.sql.SqlTypeDescriptor implementations no longer auto-register themselves with org.hibernate.type.descriptor.sql.SqlTypeDescriptorRegistry. Applications using custom SqlTypeDescriptor implementations extending the built-in ones and relying on that behavior should be updated to call SqlTypeDescriptorRegistry#addDescriptor themselves.