It's getting tougher to own an older iPhone. If it isn't slowing down because of the necessary battery throttling, it is no longer capable of running the most popular apps and games. This is the case with Pokemon GO which will no longer work on iPhones that don't support iOS 11.
Niantic, who makes Pokemon GO, says that the popular monster hunting and collecting app will not be playable on iPhones from 2013 or earlier. That means iPhone 5 and iPhone 5c (and earlier). Reason for discontinuation is the addition of new features such as enhanced AR (augmented reality) and improvements which the older hardware just can't handle, yikes.
Source: Engadget