The change in JDK-8283415 made use of the now available sealed keyword for FinalReference<T>.
Unfortunately this introduced a problem for the Espresso VM (Java on Truffle): Since Espresso is written in Java it uses the functionality of the "Host VM" to implement finalization. We do that however by introducing a new subclass of FinalReference<T> [1] which does not work anymore with the changes made in JDK-8283415. We cannot use Finalizer itself because we want to inject an additional "Guest object".
Making FinalReference<T> non-sealed would simplify things for Espresso.
[1] https://github.com/oracle/graal/blob/f195395329fba573afc6f81c5e70a18ac334dd10/espresso/src/com.oracle.truffle.espresso/src/com/oracle/truffle/espresso/ref/ClassAssembler.java#L85-L113