Xcode 16.3 not showing any PhotoKit documentation for Objective-C but it does for Swift.

look at https://developer.apple.com/documentation/photokit instead of Xcode's built-in documentation. It gives you details of the two constituent frameworks, Photos and PhotosUI, for both languages.

I can't find documentation for either in Xcode's documentation viewer, for any language.

Thanks for raising attention to this. It does look like the built in documentation in Xcode is missing information for Photos and PhotosUI. I'd recommend looking at the online documentation for the two frameworks:

https://developer.apple.com/documentation/photos?language=objc

https://developer.apple.com/documentation/photosui?language=objc

Rest assured the Objective-C is well supported.