By default, new top-level comments will only be allowed in the latest
topic from a particular set of scheduled topics. Replies to existing
comments in old topics will still be allowed - this is just intended to
prevent the cases where an old scheduled topic gets bumped back up due
to a reply and people inadvertently start adding new top-level comments
to it instead of the latest one.
This should be the correct behavior for most scheduled topics, but it
can be disabled for a particular schedule if needed.