As Samael said, Niantic has recently deprecated iOS 9 devices and Pokémon GO no longer runs on them (this will be enforced in 6 days, on February 28th 2018). We think that iOS 9/10 compatibility will be tied to how long Apple chooses to support these platforms.
If you’re planning to play on iOS, we strongly suggest going for a device that can be updated to iOS11.
Currently Pokemon go works for IOS 9 and above. HPWU is built in collaboration with Niantic so HPWU and Pokemon go will probably have about the same compatibility. By the time HPWU is released other devices might have been discontinued or they might operate on a completly different system, but either way it’s highly unlikely that anything below IOS 9 will be compatible with HPWU.