Niantic has made an announcement that the Pokemon Go would no longer be available for older iOS devices. As per the announcement, Niantic maintained that those devices, which were not upgraded to iOS 11, would no longer be able to access Pokemon Go. While making the announcement, Niantic commented that they had introduced some new features to Pokemon Go. The features according to Niantic, would not be supported in the older devices.
That said, from February 28 onwards, Pokemon Go accounts will no longer be available for those who failed to upgrade their devices to iOS 11.