/etc/network/if-up.d/wide-dhcpv6-client is in wide-dhcpv6-client 20080615-11.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/network/if-up.d/../../wide-dhcpv6/dhcp6c-ifupdown
/etc/network/if-up.d/../../wide-dhcpv6/dhcp6c-ifupdown