As part of the efforts to clean out the cruft that inevitably is left behind from semester to semester, Tstaff has closed out the course lists from previous semesters that were part of what is called a "family" in Sympa. There are still some course lists left to be cleared out, but the majority of those that were not part of the course lists for the Fall 2015 semester have been closed. For now, their content is preserved, but they will eventually be purged.
If there is an old course list that you had been using to communicate through and are now no longer able to, please email firstname.lastname@example.org, as soon as possible. The remaining old lists, which were not programmatically generated as part of a courses "family", will be looked into more closely for continued use before being closed.