Ever launch an app on your iPhone and then get a pop-up warning that says the app may slow down your iPhone? (I have old versions of certain apps, so it shows up for me every once in a while.) That warning usually appears when you’re using a 32-bit app. You can still run the app, and you probably don’t even notice the slowdown you’ve been warned about (at least in my personal experience).
Your ability to run that 32-bit app is coming to an end. As several have reported, Apple has updated the pop-up warning in the iOS 10.3 beta to say that the 32-bit app you’re running “will not work with future versions of iOS.” The warning goes on to say that the “developer of this app needs to update it to improve its compatibility.”
In October 2014, Apple announced to developers that all new apps created after February 1, 2015 must have 64-bit support. Shortly after, Apple announced that all updates to apps must also be 64-bit compatible. Any 32-bit apps submitted to Apple after June 2015 are rejected. Last September, Apple that it was going to remove any apps from the App Store that did not “function as intended, don’t follow current review guidelines, or are outdated.” Presumably, this would include apps that did not meet the 64-bit requirement.
Apple does not state which version of iOS will be 64-bit only, but since this is a major development, you can probably assume that this will happen in iOS 11. An announcement will likely be made during Apple’s Worldwide Developer Conference this summer.