Google has announced that Review extensions will stop showing this month and will be deleted entirely in AdWords accounts in February.
Advertisers that ran Review extensions and want to keep their historical data will need to export it in AdWords this month. You can find the data in both the old and new AdWords interfaces.
In the new interface (select the relevant date range):
Reviews extensions only appear on desktop devices and on the Google Search Network. Review extensions aren’t available on mobile and tablet devices or on the Display Network.
Review extensions are available in English, German, French, Spanish (including Latin American Spanish), Portuguese (including Brazilian Portuguese), Japanese, Dutch, and Italian.
Review extensions are not available for Shopping ads.