When the coursecat class was created as part of MDL-38147 it deprecated the function get_categories(). In the deprecation documentation it said that these would be an equivalent call to return all the categories on the system:

coursecat::get($categoryid)->get_children(array('recursive' => true))

coursecat::get(0)->get_children(array('recursive' => true))

However, the code for get_children never actually implemented the handling of the 'recursive' option.

EDITED: After discussion (see below) we decided not to re-implement this functionality and change phpdocs to the deprecated function instead