The forum is still available to users via the feed.php pages after the forum has been disabled by an administrator.

Repo steps:

1. Login ACP.
2. Select Board Settings under Board Configuration on the left menu.
3. Select Disable board > Yes
4. Browse to /forums/feed.php

The Disable Board function is supposed to "make the board unavailable to users who are neither administrators nor moderators" and this should include the forum feeds.