base-config was the second stage of the Debian installation process, first for the boot-floppies and later for debian-installer. I wrote and maintained it for a long time, but it's now not needed by d-i anymore, since d-i does everything in the first stage and doesn't need a second stage, so base-config has happily been obsoleted.
If you're wondering what to use to replace the old
base-config command, I
don't have a good answer really. If you're looking for a replacement to the
apt-setup command that used to be in base-config, I suggest using a copy
of the Debian mirror list and
nano /etc/apt/sources.list. apt-setup is
actually available in Debian still, in a much simplified form and currently
only as a udeb, although with some coding that could be changed..