It maybe the case that OS updates run slower on older devices but I think it's a pretty strong statement to claim that this is "intentional", at least in the sense that you seem to be implying.
Do you have some strong evidence of this? Amusingly it's actually being tested in court (see the class action referred to in the link below):
http://www.cbronline.com/news/mobility/devices/apple-accused...
Some updates actually make older devices go faster, anecdotally.
Yes, and remember that Apple intentionally writes OS updates that will run slower on their older devices, thus achieving almost the same thing but slightly less blatantly.