I have gone through the release notes for AEM Forms 6.4 release but, couldn't find anything specific to output or forms service (JEE). I am assuming that there aren't any updates for the output or forms service.
As Scott and Darren mentioned already about the output and forms service(JEE) . Service continues to provide the existing functionality with no change at JEE side .Also, at OSGI document services are getting expanded with more capabilities.
The Output (PDF) service is pretty much fully featured and now that Adapative Forms is the main focus, I expect you wont see much in the way of updates to this service. The OSGi and JEE Output services are mostly at parity as well.
The JEE core services also wont be getting many new features either. I would expect that it is only upgraded when it needs to support new OSGi features or JEE platform updates and support for new database releases. As usual there are quite a few bugfixes and patches that have been rolled into the core product.