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.

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.

Learn more about the macOS 64-bit transition