This following issue occurs when courses are set up with groups, and group messaging is activated. I have searched the tracker and can't find anyone who has reported this as a problem except in one forum post which has had no replies here: https://moodle.org/mod/forum/discuss.php?d=426485

When resetting courses to remove users and data; user enrollments, activity, and other user data are deleted. All data such as assignments, forum posts, etc are deleted as expected. 

However, group messages remain stored and so is the author's name (even if the user has been removed after a course reset). The group messages must be deleted manually by a user with permission to do so. As a solution, a teacher must be given the necessary permission to delete group messages.

There is no global setting that solves this problem nor is there an option to reset group messages.

Is there a reason that group messages posted by deleted users are not permanently removed?

This issue poses very serious GDPR problems, especially when messages contain personal data such as private conversations, phone numbers, and email addresses. I am also aware that messages can be deleted but on a message-by-message basis, this becomes a laborious task if 100s of courses contain dozens of groups that have group messaging enabled!

If it is not a bug, then it should be treated as an improvement.

The version of Moodle we currently use is 3.9.16