On an iPhone, users might instinctively swipe up to open Control Center and toggle Wi-Fi and Bluetooth off from the quick settings. Each icon switches from blue to gray, leading a user to reasonably believe they have been turned off—in other words, fully disabled. In iOS 10, that was true. However, in iOS 11, the same setting change no longer actually turns Wi-Fi or Bluetooth “off.”
Instead, what actually happens in iOS 11 when you toggle your quick settings to “off” is that the phone will disconnect from Wi-Fi networks and some devices, but remain on for Apple services. Location Services is still enabled, Apple devices (like Apple Watch and Pencil) stay connected, and services such as Handoff and Instant Hotspot stay on. Apple’s UI fails to even attempt to communicate these exceptions to its users.
Apple really doesn’t want you to turn off Bluetooth or WiFi. It’s bad design and I hope they revert this. It’s no excuse that they want their own devices (e.g. Apple Watch) to stay connected. In iOS 10, Bluetooth reactivated itself with every update. Now Apple makes sure the “normal” user isn’t turning it off at all.