In an upcoming update to Pokémon GO, developer Niantic will end support for Apple devices not capable of upgrading to iOS 11, such as iPhone 5 and iPhone 5c models. This change will take effect on February 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. Trainers will still be able to play the game using iOS 9 and above. The device must be compatible with iOS 11, but it doesn’t need to be upgraded to iOS 11 at this point.
Source: Niantic Support (@NianticHelp)