The following example shows that 'compiler.warn.sun.proprietary' diagnostics are not emitted if --system is configured to an earlier JDK version. I expected the diagnostics to be emitted for both configurations.

The diagnostics are consistently emitted for different configurations of --release.

cat A.java
class A {
  sun.misc.Unsafe theUnsafe;
}

$JAVA21_HOME/bin/java -fullversion
openjdk full version "21.0.2+13-58"
$JAVA11_HOME/bin/java -fullversion
openjdk full version "11.0.16+8"

$JAVA21_HOME/bin/javac --system $JAVA21_HOME A.java
A.java:2: warning: Unsafe is internal proprietary API and may be removed in a future release
  sun.misc.Unsafe theUnsafe;
          ^
1 warning

$JAVA21_HOME/bin/javac --system $JAVA11_HOME A.java
...
no diagnostics