Siracusa explains the reasoning behind this in a recent Hypercritical episode, and he sounds like he has some inside knowledge even if he doesn't come out and say so. It's worth a listen, but basically it goes like this: Apple wants to improve battery life on recent models. Currently the OS will switch between integrated and discrete graphics depending on needs, but going to discrete too often drains the battery. To attempt to stay on integrated graphics as often as possible, Apple has rewritten some routines to be able to avoid waking the discrete graphics. This change means that older integrated graphics are no longer supported.
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