Is the netdev group really obsolete? It seems to me that at least network-manager, wpasupplicant and avahi-daemon rely on it (for dbus.1/polkit-1). All packages add the group in .postinst.
Like with most of these old groups, the name is not optimal: it does not really deal with "devices". Only /dev/rfkill is owned by that group, and it has ACL set by (I believe) systemd-logind for the active session user. Please consider at least documenting the purpose of this group in users-and-groups.txt.
