Ever since iOS 10.1, when you open an old 32-bit app on your iOS devices you’ll be presented with a warning that the app may slow down your device and the developer needs to update it.
The apps are still fully functional, however, Apple will soon reject 32-bit apps from running on iOS. And that day may just come with iOS 11.
This is nothing new. It started with iOS 7 and iPhone 5S.
To quickly identify all the 32-bit apps installed on your iOS device, go to Settings > General > About > Applications.