There’s no supported way to open Settings to the Wi-Fi ‘page’. The fact that this worked in iOS 9 is a bug that’s been fixed in iOS 10.
For future reference, the Apple URL schemes that are officially documented for third-party use include:
Those listed in the Apple URL Scheme Reference
the specific case described in QA1924 Opening Keyboard Settings from a Keyboard Extension [This Q&A has been retired.]
Any Apple URL schemes that are not officially documented should be considered private API.
Share and Enjoy
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"