I ended up adding a blanket redirect in htaccess, so at least people get to the homepage of the site rather than a 404. Not ideal. People do click links in old newsletters.
RewriteCond %{THE_REQUEST} GET|POST\ /index.php\?option=com_acymailing\&ctrl=url\&subid=(.)\&urlid=(.)\&mailid=(.)\&Itemid=(.)
RewriteRule ^ /? [L,R]