Students can add comments to a submission at any time, even well past the cut-off date, and it appears this behaviour can't be switched off without switching off all comment functionality at a server level.

This is a problematic behaviour as students can post comments after a submission has been marked, and these may never be seen by the teacher. At our university we've had complaints about this from academics who are concerned it could affect the appeals process if, for example, a student were to post a comment notifying of a legitimate reason for failing to submit, which was not seen and therefore not taken into account by the exam board.

If Moodle behaves this way by design for some reason, it would at least be useful if it could be configured on an assignment-by-assignment basis.