Having maintained a C++ API and ABI based on implementation inheritance with virtual functions, I have deep respect for both kinds of compatibility. Someone even used placement new to put objects with vtable pointers in shared memory, expecting destructors to work in a different process!

Check out the "fragile base class problem" some time for more things than can go wrong.

Something that want made explicit: If you support closed source, shipping binaries, then the ABI can't change, or you break all compiled binaries users have installed. If rebuilding the world from source is an option, changing the ABI is an option.

(Introducing a new ABI for new subsystems is still OK - hence renaming libc when it changes)

(Say what you want about Microsoft, but COM solved that problem pretty well in the '90s...)