As described by gregor89:

We've had a similar request at our institution, but the majority of cases seem to have desired the ability to archive a forum for assessment purposes (where a grade was attached to participation within a forum and had a deadline assigned). Advising students of when the forum would be closed for new discussion / replies was necessary due to the grades attached to the activity and is similar to the logic behind displaying an assignment due date to students.

We implemented the ability to specify an open and close date for forums (and a permission allows users to post outside of these times). An alert is displayed to all users advising of the open and close dates (or when the forum became read-only). I agree with Juan that it'd be worthwhile advising users of the date a forum will (or did) become read-only.