Previewed at WWDC, launching in the fall.
Apple Reminds Developers App Updates Must Support 64-Bit Starting June 2018, Warns Customers About Unoptimized Apps
Apple already requires new apps submitted to the Mac App Store to offer 64-bit support, a change that went into effect in January of 2018, so this upcoming policy shift will only affect older apps that have not yet implemented 64-bit support.
Along with the warning to developers, Apple says customers who are running the latest macOS 10.13.4 update will begin receiving warning messages when launching a 32-bit app for the first time to let them know that the app is not optimized for their Mac. According to TechCrunch, these warnings will start at midnight Pacific Time on April 12.
With the recent release of macOS High Sierra 10.13.4, the first time users launch an app that does not support 64-bit they will see an alert that the app is not optimized for their Mac.Apple used a similar warning system when phasing out 32-bit support on iOS before eventually ending support with iOS 11, and the company has said the same plan will be used as 32-bit Mac apps are phased out.
As a reminder, new apps submitted to the Mac App Store must support 64-bit, and starting June 2018, app updates and existing apps must support 64-bit. If you distribute your apps outside the Mac App Store, we highly recommend distributing 64-bit binaries to make sure your users can continue to run your apps on future versions of macOS.
Apple first warned developers and consumers about the impending Mac App Store changes starting last June at the 2017 Worldwide Developers Conference. Apple is slowly ending support for 32-bit Mac apps and has said macOS High Sierra will be the "last macOS release to support 32-bit apps without compromises" and "all future Mac software will eventually be required to be 64-bit."