With the latest iOS 10.3 beta, the operating system has a popup every time you launch a 32-bit application, noting that the app may not work in future versions of iOS. This suggests that iOS 11 (or some version of iOS 10.x) will flat out stop support for 32-bit applications as a whole.
64-bit application support began with the iPhone 5s back in September 2013. Apple has also required developers of new apps to submit them in 64-bit since February 2015, and app updates are required to support 64-bit since June 2015.
iOS 9 included an alert notifying users that using 32-bit applications may slow performance of the device. The alert has since been updated to note that the app may stop working in future versions of iOS.
On iOS 10.2.1:
“Waterslide” May Slow Down Your iPad
The developer of this app needs to update it to improve its compatibility.
And with iOS 10.3 beta 1:
“Waterslide” Needs to Be Updated
This app will not work with future versions of iOS. The developer of this app needs to update it to improve its compatibility.
This dialog should start enticing app developers to start updating their applications to support 64-bit. Otherwise, these applications may be removed from the App Store in the future.