Previously /etc/cron.daily/apt did daily maintenance for apt. Then when Debian switched to using systemd "as a default" the apt fanboys removed non-systemd interfaces such as /etc/cron.daily/apt from the package and replaced it with /lib/systemd/system/apt-daily.timer file. This effectively broke non-systemd systems. However there isn't anything critical in the daily apt housekeeping run. Therefore even though it wasn't fully functional, for some definition of full functionality, it wasn't misbehaving that I could tell either.
Due to the backlash from this the /etc/cron.daily/apt-compat file appeared restoring the previous functionality. That is why this is in stretch-old and not stretch-new. This was aproblem in the making of Stretch 9 but seems to have been resolved favorably.In Stretch the apt package removed that file and created a systemd timer file instead.
# cat /lib/systemd/system/apt-daily.timer [Unit] Description=Daily apt activities [Timer] OnCalendar=*-*-* 6,18:00 RandomizedDelaySec=12h AccuracySec=1h Persistent=true [Install] WantedBy=timers.target