I am having the same trouble with working out how to support the following method for a plugin I help maintain.
The plugin (enrolment) has user data in two contexts USER and COURSE.
What types of contexts get added to approved_userlist and passed to this method?
Do I just need to support a course context? I wouldn't expect a userlist with user context to be passed to delete_data_for_users() as a single user deletion would be handled by delete_data_for_user()?