/etc/dhcp/dhclient-exit-hooks.d/debug is in isc-dhcp-client 4.3.5-3+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/dhcp/dhclient-exit-hooks.d/../debug
/etc/dhcp/dhclient-exit-hooks.d/../debug