As commented on https://tracker.moodle.org/browse/MDL-50332?focusedCommentId=357404&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-357404 would be great to allow teacher do decide for each enrolment instance if they want that candidate users know the reason why they can not use a that enrolment isntance (in case they can't of course, otherwise they would see the enrolment form/button or whatever)