iOS 11 it comes with a lot of changes, as you surely already found out, and one of them is related to the function for automatic brightness, auto-brightness, of the screen, this being moved by Apple from its normal position in the screen options menu.
iOS 11 displays the control button for this function in the menu Accessibility, Accessibility of the evening application, so you will have to look for it among all the existing settings to be able to control the function that allows controlling this functionality.
iOS 11 allows automatic brightness control by accessing Settings > General > Accessibility > Display Accommodations, Configurations > General > Accessibility > Display adaptations, and there you will see the button that allows you to activate or deactivate this function.
