/etc/network/if-post-down.d/openvswitch is in openvswitch-switch 2.9.2-0ubuntu0.18.04.3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/openvswitch/scripts/ifupdown.sh
/usr/share/openvswitch/scripts/ifupdown.sh