If you're using a device that runs on Android 5, iOS 10 and 11, and the iPhone 5s and iPhone 6 (even those running on iOS 12), you will no longer be able to access Pokémon Go starting October. Niantic announced it will stop supporting these devices, but the developer didn't explain why its cutting ties with the older mobile platforms and phones.
But since Pokémon Go is an augmented reality-based game, it demands more performance than regular games. Perhaps the upcoming update will prevent the game from running smoothly on these older devices.
Source: 9to5Mac