A DESCRIPTION OF THE PROBLEM :
JDK Code in places such as https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/text/DecimalFormatSymbols.java#L894-L899 is ignoring the configured SPI provider because it is not an instanceof ResourceBundleBasedAdapter. There is similar code in a few other places like DateFormatSymbols too. This renders it virtually impossible to remove the -Djava.locale.providers COMPAT option, replacing it with SPI and custom code to replicate the old JRE/COMPAT localization that doesn't match what CLDR provides.

Are there plans to enhance the SPILocaleProviderAdapter to handle all the use cases that currently require a ResourceBundelBasedAdapter before the COMPAT option is fully removed from the JDK?

In the short term, would it be possible to remove that warning when COMPAT is specified as a locale.provider? It seems 'Unfair' to constantly print that out when it isn't yet possible to remove it and retain (via rewriting) the functionality that it provides.