What does it mean if I see "security recommendation" next to a wifi network?

I recently went to a restaurant and found that their wifi network has "security recommendation" next to its name. I've never seen it before, and wonder if it might be a new thing introduced in iOS 10.

What does this mean?

It's new with iOS 10. Here's an overview of the feature by Engadget:

After connecting to an open network, iOS 10 will display a "Security Recommendation" notification beneath the network's name in the WiFi menu settings. When a user opens up the Security Recommendation, they're greeted with a notification that "open networks provide no security and expose all network traffic." There's also a recommendation to configure your router to use AES encryption for the network.