"""
This is NOT part of any supported API.
If you write code that depends on this, you do so at your own risk.
This code and its internal interfaces are subject to change or
deletion without notice.
"""

This warning has become unnecessary since JDK 9, which enforced the same idea programmatically by not exporting the respective packages.

While in most of the source code that warning acts as a visual noice, it might be reasonable to leave it in package-info.java files.