BAD NEWS: In IOS 11, turning off BLUETOOTH and WIFI doesnt really turn it off in the background. It just disconnects you from your connected bluetooth peers (or wireless networks)
The GOOD NEWS is, there is still away to totally TURN OFF your Bluetooth (and WIFI) to save power. You can do this from switching it OFF manually from your PHONE SETTINGS
Remember to always turn OFF your Radio/Bluetooth when not in use, not just to save precious battery power from your battery but to also keep your devices safe from the recent Bluetooth vulnerability known as BLUEBORNE, wherein hackers doesnt necessarily need to peer to your device to control your device and access your data (Although Apple devices are not known to be vulnerable to this, it is widely known vulnerable to all other devices. So beware.)
See how an Android device was taken control of a hacker by simply leaving its Bluetooth open.