We've found userspace ABI does change in some surprising ways occasionally but mostly it doesn't matter to anybody. e.g. we've seen the format of signal stack

We've found userspace ABI does change in some surprising ways occasionally but mostly it doesn't matter to anybody. e.g. we've seen the format of signal stack frames change in the past but nobody relies on that layout so it's OK. /proc is another one along those lines - technically somebody could probably complain if it changes but if nobody shouts then it will just drift over time.