This is happening to me also (I have iPhone 6s). I accidentally turned on ‘Screen Mirroring’ and cannot turn it off (I do not have Apple TV). I’ve done the following...
*swiped up to access control center, tapped ‘Screen Mirroring’ to turn it off, accessed ‘Screen Mirroring’ - no change.
*forced restart (several times) - no change
*pressed home key > went to settings > settings > and there is no ‘Screen Mirroring’ selection, so - no change.
-prayed, hollered, had several tantrums - no change.
HELP!!!
(Thanks in advance)