Hi jimmyfromeastland,
Thanks for using the Apple Support Communities. If you are referring to the auto-brightness feature (automatically adjusts screen brightness based on ambient light conditions), this was not changed or removed in iOS 11.2.1. If you are having issues turning this feature off, you may want to double-check that you are looking in the correct location for the setting, as outlined in the following article:
About brightness levels
iOS devices use an ambient light sensor to adjust brightness levels based on the light conditions around you. The sensor lowers brightness in dark locations and raises brightness in light locations. The auto-brightness feature is on by default.
When auto-brightness is on, you'll notice that your device's brightness slider moves according to changing light conditions.
In iOS 11, you can turn auto-brightness on or off in Settings > General > Accessibility > Display Accommodations. In iOS 10 and earlier, go to Settings > Display & Brightness.
Adjust the brightness on your iPhone, iPad, or iPod touch - Apple Support
Regards.