Took me quite a while to understand what was the function get_default_section_name() doing and how to override it in my course format.
Using self:: in a non-static function of the abstract class is very-very confusing. Besides this is clearly not needed here.

Another thing fixed is that in 2.9 the format_topics was using get_string('topic') and after MDL-51250 it was using get_string('sectionname', 'format_topics'). Although they are the same in the default en pack, I don't think it's a necessary change and may upset some users of translated sites. I reverted it back

And the last thing - the first argument type for the method was different between parent and children classes, fixed that too.