What happened to “we don't break the userspace” idea?

I think there's a reasonable interpretation where this is giving libc the tools, and libc can do something sensible with the feature without necessarily breaking everything.
The kernel is giving userspace new APIs, but not breaking any pre-existing code; taking an old system and installing this new kernel will not by itself break the dodgy memory scanning code.

Concretely, I think for Android libc could reasonably gate this on Android API level ("if your app declares targetSdkVersion >= X, libc will make use guard pages"). Every so often, the Android team increases the minimum SDK version required on their play store. So they will eventually be able to turn on guard pages unconditionally, but without taking the authors of innocent m̶a̶l̶w̶a̶r̶e anti-debug obfuscation features by surprise.