The official Pokemon GO blog has announced that the game will soon be dropping support for Apple devices not capable of upgrading to iOS 11, such as iPhone 5 and iPhone 5c models. They have explained their decision below.
“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.”