I found this statement from Linus in that thread;
"It's not that the kernel ABI (with all the system calls, all the /proc files, all the ioctl's, etc) is set in stone and "sacred". Absolutely anything can be changed, wildly.
But if it turns out that applications (or hardware) that people use end up breaking noticeably, then that is a regression."
«Really. Shut up.
The whole "no regressions" thing is very much about the fact that we don't waste users time.»
https://linux.kernel.narkive.com/7hMPh8zX/patch-revert-9fc21...
Of course in an ideal world the extent of the intended promise would be clearly written down in some obvious place. But we don't live in an ideal world.