JEP 371 introduced Hidden Classes, which at a glance seems useful to Scala. Especially since in the description it's mentioned:

If a standard API could define hidden classes that are not discoverable and have a limited lifecycle, then frameworks both inside and outside of the JDK that generate classes dynamically could instead define hidden classes. This would improve the efficiency of all language implementations built on the JVM. 

Since we are planning to update to a newer JDK at an unspecified future I hope we can figure out if this is in any way useful for us.

And if we can actually use them or whether it will be impossible for us.

Introduced in JDK 15