Apple has disappeared numerous excellent Objective-C documentation over the years. It's always worrying when good information goes away with no replacement or explanation.
The documentation situation is distressing in general right now.
I don't know what's worse: that they deprecated every single programming guide, or that there is so little communication about what they're doing and why they're doing it. Apple's developer communication and outreach is sadly lacking. So little transparency.
It seems like they think that WWDC videos are sufficient to document design principles and best practises. This is absurd, if true.