Is there official documentation for the Mach-O file format/ABI used by OSX that still exists as of December 2018?
With my previous comment, it appears that the loader.h file located at https://opensource.apple.com/source/xnu/xnu-4903.221.2/EXTERNAL_HEADERS/mach-o/loader.h.auto.html

IS the official documentation. Near the top it says

/*
 * This file describes the format of mach object files.
 */ 


And the comments in the file are more extensive than I realized.