I see this problem almost every year with macOS updates. Here are two examples that I recall of the top of my head. They are already a few years old but really annoyed me, because I spent the time to write detailed bug reports, kept re-testing the issue, and in the end Apple shipped the bugs and never fixed them:
- use of some custom NSTextAttachmentCells was broken when they added force touch dictionary lookup to NSTextView. As far as I know it was never fixed. The API is still documented and if you use it it will cause crashes as soon as someone uses force touch in a text view.
- fat binaries (32/64) stopped working when the 32bit binary uses Objective C garbage collection even when running on a 64bit system and the 64bit binary does not use garbage collection. This is clearly an error, Apple did not care about it. The only way to make such an app launchable again on 64bit is to remove the 32bit binary (that wouldn't run on a 64bit machine anyway)
- use of some custom NSTextAttachmentCells was broken when they added force touch dictionary lookup to NSTextView. As far as I know it was never fixed. The API is still documented and if you use it it will cause crashes as soon as someone uses force touch in a text view.
- fat binaries (32/64) stopped working when the 32bit binary uses Objective C garbage collection even when running on a 64bit system and the 64bit binary does not use garbage collection. This is clearly an error, Apple did not care about it. The only way to make such an app launchable again on 64bit is to remove the 32bit binary (that wouldn't run on a 64bit machine anyway)