The cldr extension needs to be converted to use the new extension registration system. More details are available on T87875.
The usage of the version constant needs to be converted to ExtensionRegistration:
https://github.com/search?q=%40wikimedia+CLDR_VERSION&type=Code&utf8=%E2%9C%93
Extension registration provides a function to check, if a specific extension is loaded or not:
ExtensionRegistry::getInstance()->isLoaded( 'extension-name' ) which returns a boolean. See the doc for more information.