As it did with iOS, Apple is moving away from 32-bit applications on its desktop platform. To this end, macOS 10.13.4, currently in beta, will warn users when they run a 32-bit app and in just a few days, Apple will actively reject programs without 64-bit executables submitted to the Mac App Store.
As Samuel Axon over at Ars Technica reports, the release notes for the 10.13.4 beta contain the following tidbit:
To prepare for a future release of macOS in which 32-bit software will no longer run without compromise, starting in macOS High Sierra 10.13.4, a user is notified on the launch of an app that depends on 32-bit software. The alert appears only once per app.
In addition, Apple's developer site posted this reminder a few days ago:
Starting January 31, new apps submitted to the Mac App Store must support 64-bit, and Mac app updates and existing apps must support 64-bit starting June 2018. 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.
As Apple remarks, this isn't just for programs distributed purely via the Mac App Store; developers releasing outside the company's ecosystem (for example, games on Steam) should provide 64-bit executables as soon as possible, lest they get caught out.