ABI doesn’t just mean calling conventions defined in things like Itanium’s spec. It also means name mangling, size of structs, and many other things. Think “if I make a shared lib and compile it, and a bunch of programs link to it dynamically at runtime, what changes to the lib can break those programs after the fact, if they don’t recompile?” The answer ends up being a lot. Again, remember that supporting new C++ features means adding things to libc++, which is linked at runtime, so you don’t want to mess it up.

Again, I don’t just mean “change the ABI of arbitrary code emitted by the compiler”, I mean the particular exported symbols in the particular implementation of libc++, which is dynamically linked. If they decide to add one extra field to a struct exported by that lib, then oops, they broke ABI. You don’t want to mess this up and you want it to be stable before letting people use it.