As you might expect, the devices in question are the oldest iPhones and iPads that can run iOS 10, namely the iPhone 5, iPhone 5c and 4th-generation iPad. All the 32-bit devices, and the last to debut with a 32-bit processor, which means that iOS 11 will be a 64-bit-only affair.
Apple has prepared developers for this transition, announcing through an iOS 10 beta earlier this year that apps will have to support 64-bit hardware in the upcoming version of iOS — that would be iOS 11 — to work on those devices.
Usually, Apple only drops support for the oldest iPhone and iPad in a new iOS release, but this time around it is including the iPhone 5c too. It launched a year after the iPhone 5, alongside the iPhone 5s, but it is likely getting dropped because it has similar hardware based on the 32-bit architecture.
