Many of the features have come to Apple’s new iOS 11 system which has seen significant changes in system control and full activation of many features within it.Some changes have been made including system settings. Of these, the option to Enable the auto-brightness seems to have disappeared.The question is How to Enable Auto-Brightness?
1-Change the screen brightness in iOS 11
Firstly, Change screen brightness from the iOS 11 console by dragging up, after that go to the screen brightness icon and drag up and down to increase or decrease screen brightness.
2-Enable Auto-brightness in iPhone X
So now it is possible to disable the auto-brightness of the screen in iOS 11. Go to the Settings, press the section General and Accessibility, then in the section Display Accommodations Enable auto-brightness.
This way you can control the brightness of the screen either manually or automatically.