Seriously. We generally release most of our new features at 10% of the users and then gradually increase the percentage on the server end. If there's any issue we can turn it off immediately rather than potentially waiting days for the app store to approve our fix.
The use of feature flags to show/hide features is arguably against the App Store guidelines [1]:
"2.3.1 Don’t include any hidden or undocumented features in your app; your app’s functionality should be clear to end-users and App Review."