iPad Multitasking support requires these orientations

iPad Multitasking support requires all the orientations but your app does not, so you need to opt out of it, just add the UIRequiresFullScreen key to your Xcode project’s Info.plist file and apply the Boolean value YES.

I am using Xamarin and there is no available option in the UI to specify "Requires full screen". I, therefore, had to follow @Michael Wang's answer with a slight modification. Here goes:

Open the info.plist file in a text editor and add the lines:

<key>UIRequiresFullScreen</key>
<true/>

I tried setting the value to "YES" but it didn't work, which was kind of expected.

In case you are wondering, I placed the above lines below the UISupportedInterfaceOrientations section

<key>UISupportedInterfaceOrientations~ipad</key>
<array>
    <string>UIInterfaceOrientationPortrait</string>
    <string>UIInterfaceOrientationPortraitUpsideDown</string>
</array>

Hope this helps someone. Credit to Michael.

In Xcode, check the "Requires Full Screen" checkbox under General > Targets, as shown below.