That being said, I've used Kindle for years because it's online etc. They have had this same feature for years on their mobile versions so I don't get how this was a killer feature of Books.
[EDIT] Two key real-book-reading behaviors this enabled, that a non-interactive page flip animation (which I personally find a ton worse than no animation) does not:
1) You could "play" with the corner and edge of the page while reading. Great for fidgeters, and analogous to what some of us do when reading real books.
2) You could start to flip the page as you were nearing the end of the current page.
But the animation is disabled by default and lately I've stuck with the regular sliding animation because it's faster and much less obtrusive. It also lets you start turning the page as you're reading the last line.
Apple has an intense culture of dogfooding, so it would surprise me very much if someone in a leadership position didn't experience the new page-flipping animation, much less explicitly approve its design.
But what I could imagine is that it fell out of product oversight and an engineer just came along and said "well that's a really complicated piece of code, I don't want to maintain that, I'll replace it with something simple", and nobody pushed back because this isn't a high-priority feature any more
That’s not really a change you miss if you do any QA of that app at all. And if it was missed that’s even worse for Apple than if it was a conscious choice.
No one has ever designed a physical book that works anything like the new page-turning approach, and you have to believe there's a reason for that.