https://www.gamespot.com/articles/pokemon-go-will-soon-no-longer-work-on-some-older-/1100-6456013/

Pokemon Go continues to receive new features through regular updates, such as the improved AR mode that rolled out for the game last month. Unfortunately for some players, an upcoming Pokemon Go update will also make it unplayable on some older iPhones. Developer Niantic confirmed that the popular mobile game will soon discontinue support for devices that cannot run the latest version of iOS.

"In an upcoming update to Pokemon Go, we will end support for Apple devices not capable of upgrading to iOS 11, such as iPhone 5 and iPhone 5c models," Niantic wrote in an update on the official Pokemon Go website. "This change is a result of improvements to Pokemon Go that push the application beyond the capabilities of the operating systems on such devices."