Niantic explained in a statement that upgrades for their games are driven on iOS version capabilities. If so, then gamers who still use the iPhone older than the iPhone 5s no longer get support for the game.
If gamers have an iPhone between 5s and X, then gamers can still play the game to capture the digital monster. As for the 5c and older models, gamers unfortunately can no longer try the famous game with Augmented reality (AR) feature.
While on the 4th generation iPad and older, then the device is no longer compatible with Pokemon GO. For those who still want to continue the game, they can switch to newer devices, which can support the iOS 11 version.