Earlier quoted context omitted.
Telling people who depended on probably internal behavior that wasn’t an explicit part of your contract to fuck off when they whine about you changing something.
You don't get to decide what is and isn't an implementation detail. And lets be real. There is no contract, and any one that is made will inevitably be broken.
Or other measure of their importance.
Does the C library change break a major distro? Or just some obscure program with three users?