/lib/live/config/0010-debconf is in live-config 5.20170112+deb9u1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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## live-config(7) - System Configuration Components
## Copyright (C) 2006-2015 Daniel Baumann <mail@daniel-baumann.ch>
##
## This program comes with ABSOLUTELY NO WARRANTY; for details see COPYING.
## This is free software, and you are welcome to redistribute it
## under certain conditions; see COPYING for details.
#set -e
Cmdline ()
{
	# Reading kernel command line
	for _PARAMETER in ${LIVE_CONFIG_CMDLINE}
	do
		case "${_PARAMETER}" in
			live-config.debconf-preseed=*|debconf-preseed=*)
				LIVE_DEBCONF_PRESEED="${_PARAMETER#*debconf-preseed=}"
				;;
		esac
	done
}
Init ()
{
	# Checking if package is installed or already configured
	if [ ! -e /var/lib/dpkg/info/debconf.list ] || \
	   [ -e /var/lib/live/config/debconf ]
	then
		exit 0
	fi
	echo -n " debconf"
}
Config ()
{
	if [ -z "${LIVE_DEBCONF_PRESEED}" ]
	then
		exit 0
	fi
	for _PRESEED in $(echo ${LIVE_DEBCONF_PRESEED} | sed -e 's/|/ /g')
	do
		case "${_PRESEED}" in
			filesystem)
				if ls /lib/live/config-preseed/* 2>&1
				then
					_PRESEEDS="${_PRESEEDS} $(for _FILE in /lib/live/config-preseed/*; do echo file://${_FILE}; done)"
				fi
				;;
			medium)
				if ls /lib/live/mount/medium/live/config-preseed/* 2>&1
				then
					_PRESEEDS="${_PRESEEDS} $(for _FILE in /lib/live/mount/medium/live/config-preseed/*; do echo file://${_FILE}; done)"
				fi
				;;
			*)
				_PRESEEDS="${_PRESEEDS} ${_PRESEED}"
				;;
		esac
	done
	for _PRESEED in ${_PRESEEDS}
	do
		_TMPFILE="$(mktemp -t live-config.XXXXXXXX)"
		if echo "${_PRESEED}" | grep -qs file://
		then
			# local file
			cp $(echo ${_PRESEED} | sed 's|file://||') "${_TMPFILE}"
		else
			# remote file
			Setup_network
			wget --quiet "${_PRESEED}" -O "${_TMPFILE}"
		fi
		debconf-set-selections < "${_TMPFILE}"
		rm -f "${_TMPFILE}"
	done
	# Creating state file
	touch /var/lib/live/config/debconf
}
Cmdline
Init
Config
 |