When we are emailing people from a group inbox, we are having
a PM conversation with them, as a support account would. In this
case mailing list headers do not make sense. It is not like a forum
topic where you may have tens or hundreds of participants -- it is a
conversation between the group and a small handful of people
directly contacting the group, often just one person.
The only header left in tact was List-Unsubsribe which is important
for letting people opt out to notifications.