Apple requires that all app functionality be described in your app's metadata and visible during review. Features that only activate after a remote flag, a specific date, or a hidden gesture violate this even if they're legitimate.
Most often triggered by a soft-launch pattern — shipping a build with a feature flag that's off during review and turned on later — or by not fully explaining a complex feature's behavior in the App Review notes.
"Your app includes hidden or undocumented features that are not accessible during the standard review process. Please ensure all features are described..."