If we want to have the old style behavior then wl_surface_set_buffer_transform is needed at the moment because Sailfish OS compositor is not handling that set_orientation_update_mask, if that would be fixed then allowing only one orientation would achieve the old style behavior. I have been planning on asking SDL developers what they think of that wl_surface_set_buffer_transform and if they would accept adding some new method or hint to their API.
Also the naming in SDL is now a bit inaccurate as the hint is still named QTWAYLAND_CONTENT_ORIENTATION when itās actually the orientation update mask.