If you can render an old thread (as opposed to pre-rendered HTML and I doubt that is done for forums), there is no storage cost or performance benefit to preven

If you can render an old thread (as opposed to pre-rendered HTML and I doubt that is done for forums), there is no storage cost or performance benefit to preventing comments on it. New comments don't need to live in the same read-only storage as the old ones. They just need to be found when the old ones are rendered, and that's easy - the new comments are in hot storage after all.