/usr/share/debhelper/autoscripts/postinst-metainit is in dh-metainit 0.0.5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 | # We assume that any previous installed /etc/init.d/#SCRIPT#
# and /etc/default/#SCRIPT# files that are conffiles are not wanted any more.
# Code taken from http://wiki.debian.org/DpkgConffileHandling
rm_conffile() {
    PKGNAME="$1"
    CONFFILE="$2"
    old_md5sum=`dpkg-query -W -f='${Conffiles}' $PKGNAME | awk '$3 == "obsolete" && $1=="'"$CONFFILE"'" {print $2}'`
    if [ -n "$old_md5sum" -a -e "$CONFFILE" ]; then
        md5sum="`md5sum \"$CONFFILE\" | sed -e \"s/ .*//\"`"
        if [ "$md5sum" != "$old_md5sum" ]; then
            echo "Obsolete conffile $CONFFILE has been modified by you."
            echo "Saving as $CONFFILE.dpkg-bak ..."
            mv -f "$CONFFILE" "$CONFFILE".dpkg-bak
        else
            echo "Removing obsolete conffile $CONFFILE ..."
            rm -f "$CONFFILE"
        fi
    fi
}
rm_conffile #PACKAGE# "/etc/default/#SCRIPT#"
rm_conffile #PACKAGE# "/etc/init.d/#SCRIPT#"
update-metainit  >/dev/null || #ERROR_HANDLER#
if [ -x "/etc/init.d/#SCRIPT#" ]; then
	if [ -x "`which invoke-rc.d 2>/dev/null`" ]; then
		invoke-rc.d #SCRIPT# start || #ERROR_HANDLER#
	else
		/etc/init.d/#SCRIPT# start || #ERROR_HANDLER#
	fi
fi
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