Apple started requiring new iOS apps to include 64-bit binaries back in iOS 8, but developers with 32-bit apps already in the App Store were allowed to stay. The writing was on the wall, however, and Apple since then has pushed for its entire ecosystem to go 64-bit, culminating with an announcement earlier this year that all apps, legacy or not, must move to 64-bit for iOS 11.
The company codified this requirement at WWDC 2017, and now that iOS 11 betas are in the hands of both developers and the public, it is reminding folks this week about the new standards for both iOS and the Mac App Store. Apple posted the reminder in two separate updates on its developer portal. For iOS 11: