Google AdWords will stop showing review extensions with ads as of mid-January 2018. All review extensions, along with their performance data, will be deleted from AdWords accounts in early February.
This change was quietly mentioned in an advertising policies help document. Google notes that advertisers must download their ad extension data before February if they wish to save it.
AdWords ad extension data can be downloaded by going to the Ads & extensions page and clicking on Extensions.
Going forward, in order to show addition information with AdWords ads, Google recommends using sitelinks, callouts, and structured snippets extensions.
This could potentially be a blow to third-party review sites, as the reviews displayed in the extensions came from non-Google websites.
There was no mention of whether or not a replacement for current ad extensions will be offered down the road.