Pretty sure the Apple search bars
used to be discoverable. When you first opened an app (like Messages), the search bar would be visible by default and you had to scroll down to hide it.
Over time, they started hiding it by default and require you to scroll up to access it.
I think that, at a general level, this principle of development is reasonable: things that used to require being extremely explicit can become more implicit over time as users become adapted to it. Computers used to feature tons of skeuomorphic design to make it obvious what everything was (so you could think about your computer with the same lens as you thought about your desk, for instance), and now we've mostly done away with that because the vast majority of users don't need it.
Where Apple fails in this regard is two things, I think:
1. They assume prior familiarity among groups who won't have it.
2. They do not leave any affordances to even suggest the presence of something hidden for those who won't know it (or have forgotten).
For the scroll bar issue, I think they could introduce an affordance in the form of, say, a couple small horizontal bars at the top of the page that kind of indicate "you can pull down on this". While early versions of this same design were pretty large and could be considered intrusive, I think users are familiar enough with the "tactile" touchscreen elements that we could develop a smaller version to use for these things.
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Your point in general still stands, though. Apple has become more and more egregious about this over time. Things used to be very discoverable, and now there's tons of hidden functionality in most of their apps (both mobile and desktop). I think they've succumbed to more feature creep over the past couple of decades than they want to admit.