As Apple warned users with iOS 10.3, iOS 11 officially drops support for 32-bit applications. Any 32-bit app will refuse to launch and instead show an alert pressuring the developer to release a 64-bit update. Older iPhones and iPads with 32-bit CPUs cannot update to iOS 11 at all.
Apple has also started the transition for macOS to drop 32-bit support. It will require new apps and updates submitted to the Mac App Store to support 64-bit apps in 2018