Pokémon Go are making some large(ish) changes to the game, specifically the app is now requiring iPhone models which can run iOS 11, the latest version of Apple’s operating system, this does mean however that older iPhones which aren’t 64-bit won’t be able to use the game anymore.
With iOS 11, Apple introduced new APIs for developers to make AR much more immersive. Prior to this, Pokémon did lack a certain depth, but the new ARKit features certainly do make the games graphics pop. But this does require the minimum operating to be iOS 11, meaning the iPhone 5 and iPhone 5c will no longer work with the game, an iPhone 5s or later is now required.
“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.”
Pokémon will stop working on the older devices from February 28 once an update is rolled out.
Source: Pokémon Go Team at Niantic