Thank you, Mayank!
When doing https://domain/webservice/rest/server.php?wstoken=&wsfunction=core_course_get_categories&criteria[0][key]=parent&criteria[0][value]=&moodlewsrestformat=json there seems to be key called "path" with an interesting value containing the category tree.
However, I am not sure if the current API allows querying on the "path" key?
When doing https://domain/webservice/rest/server.php?wstoken=&wsfunction=core_course_get_categories&criteria[0][key]=parent&criteria[0][value]=&moodlewsrestformat=json there seems to be key called "path" with an interesting value containing the category tree.
However, I am not sure if the current API allows querying on the "path" key?