https://www.theguardian.com/technology/2017/sep/21/ios-11-apple-toggling-wifi-bluetooth-control-centre-doesnt-turn-them-off

Quick switch simply disconnects phone from access points and devices rather than turning off the radios, in move criticised by security researchers

The new, redesigned Control Centre in iOS 11, which appears to allow users to toggle various settings such as turning wifi and Bluetooth off, doesn’t actually turn them completely off.