As announced by Pokemon Go on their official website, players who are using iPhone 5/5c, iPad 2, iPad mini (first generation), iPad (third generation), iPad (fourth generation), and devices still running on iOS 8, will no longer be able to play Pokemon Go, and Niantic has discontinued support for the same.
Pokemon Go players who are using an iOS system, and if the same system can’t be updated to version iOS 11 on their device, they will no longer have access to the game, starting from February 28, 2018. In addition, these players will no longer have access to their account on that device, or their PokeCoins or other items in the Bag.
Also, Niantic has ended support for all devices using iOS 8, back on November 20, 2017, and the same devices have to be updated to version iOS 9 or above.
The reason behind these changes is Apple. The company wants to push 64-bit iOS apps and the support for 32-bit apps is not available on iOS 11. In addition, all 32-bit apps previously installed, will not launch. It looks like these changes need to be done by June this year.
For more info, and if you want to see if your device is affected, visit this link and find out more.