"Precedence:" is a sendmail extension that as far as I know was only ever
implemented in sendmail and that isn't going to be standardized. Its
function is to define the PRIORITY of a message in terms of how quickly
it should be delivered. Kludges were then added to suppress delivery
errors completely or partially for various precedence levels. The author
of sendmail recommends against using the precedence levels you mentioned,
see the sendmail online manuals.
By the time AOL et al implement this, they will have switched to a
version of sendmail that generates bounces LISTSERV can parse, and you
won't see them at all.
Eric