Google announced a badge to make users aware of ad-supported apps in the "Designed for Families" program at I/O this year (see what that looks like above). Now, all apps will be subject to the same requirement going forward. Developers are getting email notifications of the change right now, and they have until January 11th to log in and state whether or not an app has ads.

Here's the text of the notice from Google.

Hello Google Play Developer,

At Google I/O, we launched “ad-supported” labels for apps in the Designed for Families program. Today, we’re announcing the expansion of this program to all apps on Google Play. Early next year, users will start to see the “Ads” label on apps that contain advertising, both inside and outside Designed for Families.

By January 11, 2016, you must sign in to the Play Developer Console and declare whether your apps contain ads (such as ads delivered through third party ad networks, display ads, native ads, and/or banner ads). After this date, the ads declaration will be required to make any updates to your apps. Please note that misrepresenting ad presence violates Google Play Developer Program Policies and may lead to suspension. You can visit our help center to learn more.

While developers are responsible for accurately declaring ad presence in their apps, Google may verify this at any time and display the “Ads” label if appropriate.

If you feel your app has been incorrectly flagged for containing ads, please contact our policy support team.

Regards,
The Google Play Team

So, there won't be any draconian measures taken if a developer doesn't update the listing in time, but any future updates will require the ad status to be included. Of course, lying about the presence of ads might lead to an app being removed. Google may also add the label to apps in some situations. The ad badge should begin appearing in the Play Store for all apps early next year.