Niantic has announced that an update to Pokémon GO that is due on the 28th February 2018 will require iOS11. This requirement means that iPhone 5 and iPhone 5c models or older and iPad Gen 4 or older will no longer be able to Catch ’em All. This also means that trainers on those older devices won’t be able to spend any PokéCoins or other premium items until they upgrade their devices to one that supports iOS11.
In an upcoming update to Pokémon GO, we will end support for Apple devices not capable of upgrading to iOS 11, such as iPhone 5 and iPhone 5c models. This change is a result of improvements to Pokémon GO that push the application beyond the capabilities of the operating systems on such devices. This change will take effect on 02/28/2018. After that date, Trainers using affected devices may no longer be able to access their Pokémon GO accounts from that device, or use their PokéCoins or other items in their Bags. They will need to switch to a supported device to continue playing Pokémon GO. You can see the full list of impacted devices and learn more by visiting our Help Center.
The decision to focus on iOS11 capable devices appears to stem from the company adding new features which require the newer OS like the recently added Apple ARKit framework support which added an AR+ mode to Pokémon GO.