> How and where, in principle, do you suppose it should (be able to) do so, given the functionality?

System logs?

> It's not as if C APIs get designed up front to return structures with a boolean "btw you called this in a nonstandard way that might fuck up in the future" flag (or, say, a pointer to a deprecation-warning structure; and then that interface has to be rock-solid stable to be of use) for every call. And if they did, nobody would ever write code that checks it.

Yeah, fair point, no-one should be using a C API at this point.