iOS12 and Mojave are being supported on a large number of devices up to 7 years old. How long are they suppose keep supporting OSes? As a software engineer, when you opt to continue supporting older devices/schemas/OS, you make design sacrifices that aren't needed if you just do a hard cutover. Building in support, even for something as simple as page syncing, so you can sync across devices to older OS versions often leads to less than optimal code and design choices. Given everything that's in iOS12 Books, it was no surprise when I saw the warning about syncing.
It was the same thing with iOS 9 and Notes which was not backwards compatible and it was documented quite a bit as well as on the Apple's support site.
I know. I'm not talking about supporting devices with the operating system. Obviously they can't be supported for ever, there are technical limitations to overcome, CPU architectures progress etc. But there is an incongruity between the devices that will support reading and buying a book, and the devices that can support syncing the page location. So for example if I buy a book on a device that won't support iOS12, I can quite happily do that and read it, it just won't sync the current page with an iOS12 device. This doesn't seem like an advanced feature, this seems like table stakes in the ebook business to me. Amazon manage to keep location syncing working between devices many years older and across many different OS's.
Apple want to get into services, well people like their services to keep working dependably day in day out, not suddenly and seemingly arbitrarily stop working because they've upgraded one of their devices.
I'm not asking for my 7 year old device to run the latest and greatest, I'm just asking for it to sync a simple location in a book. That should not be beyond the ability of the richest company on the planet. The fact that it is, seems to be pretty contrary to Apple's stated aim of delighting their customers. As someone with a computing background, I can understand that maybe it's down to a changed data structure, or maybe an engineer hasn't got round to implementing backward compatibility yet, perhaps it's on their development roadmap. But the vast majority of Apples customers don't have that background, they won't understand, all they'll want to know is why their page location doesn't sync anymore when it did yesterday.