I tried to pass false but it doesn't seem to do anything. In any regards what I was really looking for was to unread / unseen. At this point it was mostly for testing purposes but I could see this becoming a feature in our system.
mark_read Comma separated list of activity ids to mark as read, or boolean true
mark_seen Comma separated list of activity ids to mark as seen, or boolean true
$results = $notification_feed->getActivities(0, 10, $options);
Marking activities as unread or unseen is currently not supported by Stream's latest API (v1.0)