[andrewaylett]

The OP said old devices, but in response to someone complementing support for old software -- you can see where the change in topic might lead to difficulties in communication?

Unfortunately, board support packages from the system-on-chip manufacturer limit kernel upgrades. Even then, Project Treble should make it easier to upgrade to newer versions of higher-level components. But Treble was introduced with Android 8.0, so while newer phones should be able to be upgraded more easily, that doesn't help hardware of the era you're referring to.

In any case, the problem isn't with app developers and older versions of Android -- although I'm happy that many try to mitigate the hardware vendors' lack of support. It's that phone hardware is insufficiently open or standardised (in contrast to x86) meaning that OS vendors can't support it.