Credits for this solution go to YUF in this thread on the Apple Developer forum:
It uses MKMapCamera to determine the zoom level, not setRegion.
MKMapCamera* camera = [MKMapCamera
cameraLookingAtCenterCoordinate:(CLLocationCoordinate2D)centerCoordinate
fromEyeCoordinate:(CLLocationCoordinate2D)eyeCoordinate
eyeAltitude:(CLLocationDistance)eyeAltitude];
[mapView setCamera:camera animated:NO];
If you keep centerCoordinate and eyeCoordinate the same, the camera will look straight down. The altitude will give you control over the zoom. It won't go all the way down to zero, but it will give equivalent zoom levels as on previous iOS versions.