Main Menu block non-js dropdowns are deprecated error

As reported by alrachels in a forum discussion Main Menu block non-js dropdowns are deprecated:

On the QA Testing site logged in as administrator on the front page with debug on, add a Main Menu block. Configure the block to display throughout the entire site. Go to a course such as Marvelous Media Players. You will see a debug message:

non-js dropdowns are deprecated.

    line 339 of /course/renderer.php: call to debugging()
    line 287 of /course/renderer.php: call to core_course_renderer->course_section_add_cm_control_nonajax()
    line 157 of /blocks/site_main_menu/block_site_main_menu.php: call to core_course_renderer->course_section_add_cm_control()
    line 766 of /blocks/moodleblock.class.php: call to block_site_main_menu->get_content()
    line 235 of /blocks/moodleblock.class.php: call to block_list->formatted_contents()
    line 1181 of /lib/blocklib.php: call to block_base->get_content_for_output()
    line 1239 of /lib/blocklib.php: call to block_manager->create_block_contents()
    line 374 of /lib/blocklib.php: call to block_manager->ensure_content_created()
    line 3980 of /lib/outputrenderers.php: call to block_manager->region_has_content()
    line 40 of /theme/boost/layout/columns2.php: call to core_renderer->blocks()
    line 1396 of /lib/outputrenderers.php: call to include()
    line 1326 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
    line 245 of /course/view.php: call to core_renderer->header()