@paolo phplist has always had a List-Unsubscribe header. Just look at the headers of any email sent from phplist.
Since release 3.4.9 (last December) it also has a List-Unsubscribe-Post header which seems to increase the likelihood of a mail client showing an “Unsubscribe” button. On my Android phone the Yahoo mail app and the gmail app now both show an Unsubscribe button.