The get_output_classname method is used to invoke overridden output
components from course_formats. Now the method does not accept null
output names anymore, and it controls the format class is extending
the core one.
Until Moodle 4.0, renderer.php file was optional (although highly recommended)
for course formats. From Moodle 4.0 onwards, renderer is required to support
the new course editor implementation.
The legacy_format_renderer class has been created for backward compatibility,
to avoid some errors with course formats (such as social) without the renderer
file. Apart from that, course_format->get_renderer() method has been reviewed
to use this legacy_format_renderer when no renderer.php file is found.
Ferran Recio