ADDITIONAL SYSTEM INFORMATION :
java version "1.8.0_441"
Java(TM) SE Runtime Environment (build 1.8.0_441-b07)
Java HotSpot(TM) 64-Bit Server VM (build 25.441-b07, mixed mode)

A DESCRIPTION OF THE PROBLEM :
When we call an internal library (for example: com.sun.org.apache.xerces.internal.dom.DeferredDocumentImpl), JDK 8 compiles with warnings and is possible to execute the program. In JDK 11 and above, it shows compilation errors.

To keep JDK consistency between versions, JDK 8 should throw the errors during compilation.

STEPS TO REPRODUCE THE PROBLEM
Compile test.java file

javac test.java

with JDK 11 and above, it will fail. With JDK 8 is possible to successfully run the code

java test

ACTUAL VS EXPECTED BEHAVIOR
EXPECTED -
(as in JDK 11)

test.java:1: error: package com.sun.org.apache.xerces.internal.dom is not visible
import com.sun.org.apache.xerces.internal.dom.DeferredDocumentImpl;
                                         ^
  (package com.sun.org.apache.xerces.internal.dom is declared in module java.xml, which does not export it)
1 error

ACTUAL -
(as in JDK 8)

test.java:1: warning: DeferredDocumentImpl is internal proprietary API and may be removed in a future release
import com.sun.org.apache.xerces.internal.dom.DeferredDocumentImpl;
                                             ^
test.java:5: warning: DeferredDocumentImpl is internal proprietary API and may be removed in a future release
    DeferredDocumentImpl d = new DeferredDocumentImpl();
    ^
test.java:5: warning: DeferredDocumentImpl is internal proprietary API and may be removed in a future release
    DeferredDocumentImpl d = new DeferredDocumentImpl();
                                 ^
3 warnings

---------- BEGIN SOURCE ----------
import com.sun.org.apache.xerces.internal.dom.DeferredDocumentImpl;

public class test {
   public static void main(String[] args) {
        DeferredDocumentImpl d = new DeferredDocumentImpl();
   }
}
---------- END SOURCE ----------