The design choices have to be made to artificially shorten the life, not for any other purpose. For example, I can make a device cheaper and use cheaper materials that wear out faster - that's not planned obsolescence. However if I include a little plastic tab that I know will break before everything else for no good reason other than making it break faster - that is.
I suspect you'll have a hard time arguing compellingly that the purpose of apples closed update system was to shorten the useful life, rather than as a side effect of other product and design goals.
Phones are sort of a bad example, because for a long time the replacement cycle was driven by actual technical obsolescence, arguably only relatively recently has this ceased to be the case.
So what is the reason to prevent updates of software after the support is over? This is a classical software anti-feature.
The reason 1st gen iPads don’t have this is fairly obvious though - the unlocking feature is itself a likely vector for attacks e.g. by law enforcement unlocking tools, etc.
I’m not sure what it would take to enable it to be done securely without destroying the overall security model but it’s clearly not trivial, and would take some investment that certainly wasn’t possible at the time the 1st gen device was released.