Compatibility of oldish iPhones, Apple's policy, future perspective I'm shopping for a used iPhone. I would not use it as a personal phone, just as a device to explore Apple's apps, for professional reasons. 

I wonder what older models are still compatible with recently launched apps developed for new devices.

Is there some official road map about support from Apple and back compatibility policy?  How far back in iPhone version can I safely go to buy a cheap device that will last at least one year?
Here's the list of iPhones that currently support iOS 11:


iPhone 5S
6
6 Plus
6S
6S Plus
SE
7
7 Plus
8
8 Plus
iPhone X


With the exception of the last 3, you can find reasonably inexpensive (probably not 'cheap') used iPhones that should last for another 1-3 years.  Apple doesn't post roadmaps as to upcoming devices or the iOS versions that support.