Extensions that call PyObject_GC_UnTrack before calling Py_TRASHCAN_BEGIN will still work, they will just take a very minor performance hit.  I don't think it is worth the trouble to introduce new macros for that reason.  Extensions that really care about performance can wrap the call in a Python version ifdef.

There is an issue if someone writes and tests their extension with the new API, i.e. without having the explicit PyObject_GC_UnTrack() call in their dealloc method.  If they compile with an older Python, they likely get a crash.  If they compile with asserts enable, they would get an assert fail in _PyTrash_thread_deposit_object, i.e.:

    _PyObject_ASSERT(op, !_PyObject_GC_IS_TRACKED(op));

I guess that's an argument for new macros.