Been wanting this feature for awhile. Jumped right in and found it working perfectly with new iPad Pro and only allowing screen shots (not screensharing) with older iPhone 6. Both devices running IOS 11.
Anyone else experiencing this?
I've just tried out the screen sharing feature for iOS 11 in TeamViewer QuickSupport.
It worked for me on an iPad Pro 12.9 (1st gen), iPhone 6 Plus and iPod Touch (6th gen), all running iOS 11, so I can't think of an obvious reason why it wouldn't be available on your iPhone 6.