What do you mean constant API changes? GTK 3 existed for what, 10 years? GLib hasn't had an API break in how long?
Linux devs will ignore a feature going obsolete and complain 10 years later when it's removed. It doesn't matter how long you give because they were never actually working on it in the mean time.
GLib 2.0 was released on 2002-03-08. There's not a whole lot you'd be using on the desktop that's maintained ABI for longer; mainly X11 libraries, glibc, NSS, zlib, libxml2 (latter one is also a GNOME project).
(Anything using C++ would have a libstdc++.so.6 related ABI break at a later date.)