/usr/share/initramfs-tools/scripts/local-bottom/ifdown is in dropbear-initramfs 2016.72-1.
This file is owned by root:root, with mode 0o755.
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 | #!/bin/sh
PREREQ=""
prereqs() {
    echo "$PREREQ"
}
case "$1" in
    prereqs)
        prereqs
        exit 0
    ;;
esac
[ "$DROPBEAR_IFDOWN" != none ] || exit 0
for IFACE in /sys/class/net/${DROPBEAR_IFDOWN:-*}; do
    [ -e "$IFACE" ] || continue
    IFACE="${IFACE#/sys/class/net/}"
    ip link    set   dev "$IFACE" down
    ip address flush dev "$IFACE"
    ip route   flush dev "$IFACE"
done
 |