When Apple offers an operating system upgrade to their mobile devices without allowing the ability to revert back to the previous version (beyond the few day window after initial release), when the upgrade degrades performance, when a security fix is added to both the current version and newer version of the OS but requiring devices capable of update to update, could certainly justify the obsolescence claim. If you take issue with the use of the word "obsolescence" what word would you use for such behavior?
That's a very good point, and Apple are certainly guilty of this. The performance of some older devices running current software would be unacceptable to me, to the point where I couldn't stomach using them. However, the majority of them still work as intended, albeit slowly and sometimes less stable.
It's a tricky one. Update older devices with security fixes and the latest OS upgrade minus new, key features, and degrade performance. Or, completely abandon older devices, leave security flaws vulnerable, albeit to the benefit of the performance of older devices.
There's no correct answer here unfortunately, but there's two solutions that Apple conveniently ignores - as you say, the ability to downgrade OS version. Alternatively, Apple could still issue security fixes for older OS's without the need to upgrade. Unfortunately, Apple aren't interested in doing either of these things, so obsessed are they at massaging the numbers so it appears the majority of their active, sold devices run the current OS. I see how their inaction on these two points could be construed as planned obsolescence.