So does Glibc in its stdio implementation (bog-standard vtables except the vtable pointer can occasionally change at runtime), Linux in a number of places (I don’t really know much there), the FreeBSD kernel in its driver ABI (more like ObjC than C++) and many others.

(GObject is of course also object-oriented, but IMO hardly counts as C programming in how it works, it’s more of a separate OO language hand-translated into C—Vala is basically that language, finally implemented years later.)