Answer by dessert for How to change cron log level?

From Ubuntu 15.04 on, upstart using /etc/init/*.conf is replaced by systemd using configuration files in /lib/systemd/system/ and /etc/systemd/system/. Although a file /etc/init/cron.conf is still existent in Ubuntu 16.04, the script normally in use to start cron now is /lib/systemd/system/cron.service. If you want to add extra options, edit this file with

sudo systemctl edit --full cron

and replace the line

ExecStart=/usr/sbin/cron -f $EXTRA_OPTS

by e. g.

ExecStart=/usr/sbin/cron -L 15 -f $EXTRA_OPTS

To reload the configuration, run sudo systemctl restart cron or just reboot. You can test what exact command a service was started with using systemctl status, e. g. for cron (see last line):

> systemctl status cron● cron.service - Regular background program processing daemon   Loaded: loaded (/lib/systemd/system/cron.service; enabled; vendor preset: enabled)   Active: active (running) since So 2017-08-27 09:56:18 CEST; 1h 15min ago     Docs: man:cron(8) Main PID: 26021 (cron)   CGroup: /system.slice/cron.service└─26021 /usr/sbin/cron -f