Is described in https://lists.debian.org/debian-devel/2014/12/msg00368.html
- with systemd /var/run/motd.dynamic is not written cause empty motd.service (see also here)
- pam_motd provides a facility (via a custom patch, brought over by Ubuntu) that run-parts /etc/update-motd.d, this directory needs to be created on Debian
- pam_motd writes the run-parts output to (/var)/run/motd, but that's not used by anything, Ubuntu's pam_motd is patching this to be /run/motd.dynamic