I don't see how Apple could do this without people uncovering it in the code of their updates.
I think the real issue is that Apple holds back hardware specs, and then when updates come to a 1 or 2 year old device, the device may not be able to handle it -- slowing down, glitchy, etc. And so people get the impression Apple intentionally cripples their devices to "force" upgrades.
Agreed. I don't think that Apple intentionally slow down old models. It would have come out by now.