Apple has long made it clear that support for 32-bit apps will be phased out eventually. On the iPhone and iPad side, the final nail in the coffin comes with iOS 11. However, macOS developers have a bit more time left to prepare for a 64-bit only future.
Starting next year, Apple will no longer accept 32-bit macOS apps in the App Store, which means that any new titles must be 64-bit from January onwards. However, for existing apps and their upcoming updates there is a different deadline.
Apple has revealed that macOS High Sierra, which it just previewed at WWDC 2017, will be the "last macOS release to support 32-bit apps without compromises," and that its successor will "aggressively" alert users when they run 32-bit apps. We can only presume that the macOS release that arrives in 2019 will actually drop support for 32-bit apps.
On the iOS side, there were nearly 200,000 apps not ready for iOS 11 in March, 2017. Apple stopped accepting 32-bit apps since January 1, 2015 and stopped approving 32-bit updates since June 1. However, since the deadlines for macOS will not have an effect until next year, and developers are not pressed to update their apps right away, it is too soon to tell how many macOS titles could be affected by Apple's decision.