/etc/inittab is not used under
systemd any longer. If you wanted, you could install
systemv and you would find yourself a brand new
inittab, but this would mean walking backward like crabs.
You can instead edit the file
/lib/systemd/system/getty@.service and change the line
ExecStart=-/sbin/agetty --noclear %I $TERM
ExecStart=-/sbin/agetty --noclear -a root %I $TERM
This just follows from the agetty manual page, which states, among other things:
Log the specified user automatically in without asking for a login name and password. The
-f username option is added to the
/bin/login command line by default. The
--login-options option changes this default behavior and then only
\u is replaced by the
username and no other option is added to the login command
To be sure, I just tested this on my Debian VM, and it works fine.