> there is no obligation to keep things working for the.
You opened with the correct observation that APIs are mostly for people. Saying there is no obligation here contradicts the expected social norms. And even more importantly, intent does not tightly correspond with reality, and what can happen, tends to happen. The actual code actually existing always has the final say. If you intend to have the best outcome for everyone involve, conform to the unalterable realities as much as possible - if the interface should be public, make it public. If the interface should be private, make it private.
I specifically said "when tools are not expressive enough to capture and enforce intent".
Suppose you're writing a library in Python. Everything in it is public. Even the dunder class members are, because it's just name mangling, and the language spec even documents what exactly it does!
Now, is anyone going to seriously claim that every single identifier in every Python library is part of its public API, and any change that affects it is a breaking change? Because that's certainly not the "expected social norm".
Granted, Python is a somewhat extreme example. But in practice, this also comes up in languages like Java and C#, when dependencies are more intricate than what the access control system in those languages can fully express.
And then there are backdoors:
> What can happen, tends to happen. The actual code actually existing always has the final say.
You can use Reflection to access any private field of any object in Java. There's actual existing code doing that in practice, too. Does it have the final say, and does it mean that internal representation of any Java class in any shipped Java library has to be immutable, so as to not break the API clients?
You opened with the correct observation that APIs are mostly for people. Saying there is no obligation here contradicts the expected social norms. And even more importantly, intent does not tightly correspond with reality, and what can happen, tends to happen. The actual code actually existing always has the final say. If you intend to have the best outcome for everyone involve, conform to the unalterable realities as much as possible - if the interface should be public, make it public. If the interface should be private, make it private.