With iOS 11, Apple is phasing out support for 32-bit apps and devices completely from iOS. On the macOS side, Apple says that macOS High Sierra will be the last release of the OS to support 32-bit apps.
The company revealed this detail during its Platform State of the Union keynote where it said that High Sierra will be the “last macOS release to support 32-bit apps without compromises.”
Starting from January 2018, all new apps being uploaded to the Mac App Store must be 64-bit, with all apps and updates being submitted from June 2018 also require adhering to the same requirement. Apple will also start “aggressively” informing users about using 32-bit apps on macOS starting with the release after High Sierra.
This is similar to what Apple did with 32-bit apps on iOS. It first started warning users about using 32-bit apps with the release of iOS 10.3, and with iOS 11, it has completely phased them out and even dropped support for them from the App Store. By the time iOS 11 release, developers are required to update their legacy 32-bit iOS apps or their listing will be removed from the store.