Comment 20 for bug 1637800

+ [ -d /run/systemd/system ]
+ systemctl --system daemon-reload
+ deb-systemd-invoke start motd-news.service
motd-news.service is a disabled or a static unit, not starting it.
+ [ -d /run/systemd/system ]
+ systemctl --system daemon-reload
+ deb-systemd-invoke start motd-news.timer
motd-news.timer is a disabled or a static unit, not starting it.

The xenial package is not using debhelper, and the manual entries in postinst do not enable it, just try to start it.

This is the bit that dh_systemd_enable adds to postinst in bionic, for example:
# Automatically added by dh_systemd_enable/11.1.6ubuntu2
if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ] ; then
        # This will only remove masks created by d-s-h on package removal.
        deb-systemd-helper unmask 'motd-news.timer' >/dev/null || true

        # was-enabled defaults to true, so new installations run enable.
        if deb-systemd-helper --quiet was-enabled 'motd-news.timer'; then
                # Enables the unit on first installation, creates new
                # symlinks on upgrades if the unit file has changed.
                deb-systemd-helper enable 'motd-news.timer' >/dev/null || true
        else
                # Update the statefile to add new symlinks (if any), which need to be
                # cleaned up on purge. Also remove old symlinks.
                deb-systemd-helper update-state 'motd-news.timer' >/dev/null || true
        fi
fi

And then come the deb-systemd-invoke start bits that were added manually to the xenial package.