Apple’s attitude here is that it’s an inherent risk of using private APIs, because it’s not something they want devs doing. They don’t facilitate it like MS tends to. Don’t touch the stove if you don’t want to get burnt.
If I were building a FOSS platform, I wouldn't give a second thought to third parties making use of my platform's private APIs. They're private for a reason, whether that be because they're not yet fully baked or because using them can have unintended consequences, they're not intended for public consumption. I especially wouldn't want somebody else's platform to depend on my private APIs, because I am then effectively locked into keeping that API frozen in time by the numerous others building on this other person's platform.
It's generally poor practice to build upon such brittle things as under-the-hood tinkering anyway.
The alternative doesn’t help either. That’s the approach Microsoft has taken and look what a mess Windows is because of it.
Can you point at any part of windows being a mess specifically because of backwards compatibility?
Software A an B works fine before update, after update, both breaks => whatever changed is the problem(in this case, the OS).