As noted by MacRumors:

The second beta of iOS 16.4 that was introduced to developers today appears to have a limited number of new features, but it does have a major update for those who use Apple Books - it reintroduces an option for the page-turning animation. Source: iOS 16.4 Beta 2 Re-Adds Page Turning Animation to Apple Books - MacRumors

One quick question: who decided it was a good idea to remove such page-turning animation in the first place? What problem did it solve? Books are still a big part of our life, and this animation is a great reference to the experience of reading a book.