The big issue with this is apple does not own all of the IP in a any of their hardware, much of it is a mixture of licenses from many vendors some of it owned by apple much of it just licensed.

Apples licenses for these will include very strict NDA clauses that make it impossible for apple to share the documentation they are provided but that vendor.

Apple has moved much of the stack completely in-house, that point is relatively moot.

So those NDAs become void. Excellent.

Isn’t a warranty of merchantability thing? If something is described as doing X, and you take X away, you’ve taken value from my property.

Which makes an excellent case for mandatory docs.

I don't want my speakers to stop working because I stopped paying subscription.

As long as they are "your" speakers. Subscription sounds like you're renting speakers. I know that's probably a facetious/pedantic point but it strikes to the heart of the fact that we should be calling for more accurate description of the relationship between a consumer and the hardware they acquire i.e. calling a spade a spade. You're not buying a laptop, or an M1 CPU in a metal case. You're buying a magic Apple service delivery box that happens to permit certain other uses. Customer expectations should match description should match function.

I say this as a longstanding iPhone user, and up until recently, Mac user.

Not a legal obligation but a support obligation. Documenting implies stability of an API. It says, use this thing we intend for you to use. Usually, you mark an API as deprecated, and often provide an alternative, before removing it.

Sometimes backwards compatibility is preserved even to the point of preserving bugs (especially if the companies/apps depending on it are large):

https://www.triplefault.io/2017/07/breaking-backwards-compat...

https://arstechnica.com/gadgets/2008/11/ars-investigates-doe...