iOS 9 : Warning “All interface orientations must be supported unless the app requires full screen” for universal app

The solution to this is to use "Device Specific Keys": https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/AboutInformationPropertyListFiles.html#//apple_ref/doc/uid/TP40009254-SW9

Your plist values would therefore look something like:

<key>UISupportedInterfaceOrientations</key>
<array>
    <string>UIInterfaceOrientationPortrait</string>
    <string>UIInterfaceOrientationPortraitUpsideDown</string>
</array>
<key>UISupportedInterfaceOrientations~ipad</key>
<array>
    <string>UIInterfaceOrientationPortrait</string>
    <string>UIInterfaceOrientationPortraitUpsideDown</string>
    <string>UIInterfaceOrientationLandscapeLeft</string>
    <string>UIInterfaceOrientationLandscapeRight</string>
</array>
<key>UIRequiresFullScreen</key>
<true/>
<key>UIRequiresFullScreen~ipad</key>
<false/>

When I remove the iPad specific version of the UIRequiresFullScreen key, I lose the full split-screen functionality - only "slide over" is available because that doesn't affect my app's use of the full device screen.

The "Device Orientation" checkboxes are for the default plist values. The only way they wouldn't affect the app on the iPad is if there's a more specific value in the plist, therefore a value specifically for iPad.

When the system searches for a key in your app’s Info.plist file, it chooses the key that is most specific to the current device and platform.