Uninstallation of an authentication plugin is being prevented if there are users who have been created using that plugin. Currently also deleted users prevent the uninstallation. Shouldn't the feature take only active users into consideration?
The function is_uninstall_allowed() in lib/classes/plugininfo/auth.php should be changed to ignore deleted users.
See discussion here: https://moodle.org/mod/forum/discuss.php?d=272751