With iOS 9, developers can cutoff younger devices in a way that was not previously possible. Although iOS 9 runs on every device that runs iOS 8, app developers are free to specify more restrictive compatibility requirements.
In fact, with iOS 9, developers can choose to make their apps exclude any non-64 bit architecture. This means all iPod touch models, all iPhones before the iPhone 5s and all iPads before the iPad Air will not be able to install apps where developers have required 64-bit CPUs.
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Apple started using 64-bit CPUs with the A7 in the iPhone 5s, a 2013 device. Developers often complain about supporting older hardware platforms (such as the A5 which is very slow by modern standards) and so this option will offer a quick-and-easy way to simplify development.
If a developer enables this option, customers browsing the App Store with older devices will simply not see that app in the Store and won’t be able to download or install it.
The limit has little to do with the 64-bit support itself — binaries can be built that support both platforms. However, the processors are simply much more powerful than their predecessors which puts a lot of strain on developers to support both platforms well. Apple showed how GPU performance increased dramatically in the last two years with the 64-bit processors (above).
It’s a highly convenient cutoff for developers to make it easy for them to drop compatibility for older devices. Obviously, customers with non-64 bit iPads and iPhones may be upset as developers may actively cut off support in their apps for these hardware variants due to the convenience, not necessarily for technical reasons.
As developers like making their lives easier, with iOS 9, users may find their devices become obsolete faster as more apps no longer support them.