I compared country and region names of Locale.ROOT and Locale.US.
On default, Locale.ROOT comes from JRE, Locale.US comes from CLDR.

Testcase and output is as follows
=========================
$ cat ISO3166.java
import java.util.*;

public class ISO3166 {
    public static void main(String[] args) {
        for (String s : args) {
            Locale loc = new Locale.Builder().setRegion(s).build();
            String root = loc.getDisplayCountry(Locale.ROOT);
            String us = loc.getDisplayCountry(Locale.US);
            System.out.printf("%s: %s [%s<=>%s]%n", root.equalsIgnoreCase(us) ? "=" : " ", s, root, us);
        }
    }
}
$ ~/jdk-16.jdk/Contents/Home/bin/java -version
openjdk version "16-ea" 2021-03-16
OpenJDK Runtime Environment (build 16-ea+20-1092)
OpenJDK 64-Bit Server VM (build 16-ea+20-1092, mixed mode, sharing)
$ ~/jdk-16.jdk/Contents/Home/bin/java ISO3166.java BQ SZ
 : BQ [Bonaire, Sint Eustatius and Saba<=>Caribbean Netherlands]
 : SZ [Swaziland<=>Eswatini]
=========================

According to following URL, it seems country and region names are updated.
https://github.com/eggert/tz/compare/2012e..master#diff-e5919fb2a195c61372529aa1b85af27218bd019942ea35bd1faf1fb746209560

Also, see:
https://www.iso.org/obp/ui/#iso:code:3166:BQ (I don't know which one is valid)
https://www.iso.org/obp/ui/#iso:code:3166:SZ (Country name was changed)