Pokemon GO ends support for these iPhones and Android smartphones1
If you're using an older iPhone or Android device, you will no longer be able to play Pokemon GO from October. In an upcoming update to Pokemon GO coming in October, Niantic will end support for devices running Android 5, iOS 10, and iOS 11. Apart from these devices, Niantic announced that iPhone 5s and iPhone 6 devices will no longer be supported after the October update gets released.
Truthfully, there shouldn't be too many people still using Android 5, iOS 11, and iOS 11 handsets to play Pokemon GO. Those who still cling to these very old devices for whatever reasons will have to switch to newer phones if they want to keep playing their favorite game.
It's not unusual for developers to discontinue support for older devices as they redirecting their resources to making their apps and services run better on modern products. If you're one of the people affected by this change, you have a bit more than a month to switch to another phone that will still be able to run Pokemon GO after the October update.