Users now see a warning message if they specify either `COMPAT` or `JRE` locale data with the `java.locale.providers` system property and call some locale-sensitive operations. `COMPAT` was provided for migration to the `CLDR` locale data at the time of JDK 9, where it became the default locale data ([JEP 252](https://openjdk.org/jeps/252)). JDK 21 retains the legacy locale data of JDK 8 for compatibility, but some of the newer functionalities are not applied. The legacy locale data will be removed in a future release. Users are encouraged to migrate to the `CLDR` locale data.