In an announcement on August 30, Apple told app developers that they would have to have a privacy policy on their apps by October 3, or they would not be able to be sold in the App Store:

Upcoming privacy policy requirement.

August 30, 2018
As a reminder, in June the App Store Review Guidelines were updated to require a privacy policy for all new apps and app updates as part of the app review process. Starting October 3, 2018, App Store Connect will require a privacy policy for all new apps and app updates before they can be submitted for distribution on the App Store or through TestFlight external testing. In addition, your app’s privacy policy link or text will only be editable when you submit a new version of your app.

To add or edit your privacy policy for the App Store:

Go to My Apps in App Store Connect, and click on your app.
Under App Store, click on App Information.
In the top right corner, add your privacy policy link for iOS apps or macOS apps, or enter text directly for tvOS apps.
Click Save.

To add your privacy policy link to your app for external TestFlight distribution:

Go to My Apps in App Store Connect, and click on your app.
Under TestFlight, click Test Information.
Add your privacy policy link for iOS apps, or enter text directly for tvOS apps.
Click Save.

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Presumably most apps already have a  privacy policy, but this is surely a sign that the internet’s lowest common denominator is moving upward.