I have the /usr/lib/systemd and /etc/systemd directories, and systemd is running.
But I also have the /etc/init/ directory, with several .conf configuration files, I thought this was a sign of Upstart.
And I also thought Debian had moved to System V starting release 7. But that would imply having a /etc/inittab (that I don't have) directory, wouldn't it?
/etc/init is indeed where Upstart job files are located. However you do not have Upstart itself on Debian 9, and no job files there are actually being used.
Such job files are installed by the various packages that include their own Upstart job definitions, just as they may install van Smorenburg rc scripts or systemd unit files that your particular choice of service/system management might not use.
In August of this year, some Debian people proposed a coördinated effort to remove all upstart job files from all Debian packages.
/etc/init, which the questioner here is questioning the presence of. Ironically, there is an answer about/etc/initthat involves recent discussions in the Debian world, and an answer to this question could mention that. It has nothing to do with/etc/inittab.