This may be seen on any iOS device running version 10 and newer. This is an additional security warning that will be seen when attaching to an open or public SSID but will also be seen on a Guest Splash or Guest Portal that requires an acknowledgement page and/or a specific user login. This would also be seen on any vendors network that is unsecured.
What you will see on the iOS device looks like the screen shots below. Tapping the information icon will bring up the message on the right: