14.19 The synchronized Statement of the JLS for 401 [1] states:


SynchronizedStatement:
    synchronized ( Expression ) Block

The type of Expression must be a reference type, and must not be a final value class type, or a type variable or intersection type bounded by a final value class type, or a compile-time error occurs.

javac is not allowing value classes even if they are abstract, javac should be in sync with the latest spec

[1] https://cr.openjdk.org/~dlsmith/jep401/jep401-20240222/specs/value-objects-jls.html