Is it possible to opt your iPad app out of multitasking on iOS 9

To opt-out (disable) multi-tasking for your application:

Select your Target → General Section → Scroll Down and check Requires full screen

It gets applied to the plist's UIRequiresFullScreen key value.

Note: Apps are required to support all screen orientations if it supports multitasking. Otherwise, apps will get the following error on publishing:

Invalid Bundle. iPad Multitasking support requires these orientations: ...

You have to modify your project to support multitasking. According to WWDC 2015 video, to adopt your app for multitasking, satisfy these requirements:

1. Build your app with iOS 9 SDK
2. Support all orientations
3. Use Launch Storyboards

So, if any of this is not done yet, your app will not be able to support multitasking.

Of course, if you don't use size classes, put it at the top of the list.

Edit: according to you question edit. There is a UIRequiresFullScreen key in Info.plist. See more at Apple docs