The tag is present in Lintian version 2.104.319. That is the most recent version we know about.
/etc/init.d script seems to be sourcing an
/etc/default/ file without checking for its existence first.
/etc/default/ can be deleted by the administrator at
any time, and init scripts are required to handle the situation
gracefully. For example:
[ -r /etc/default/foo ] && . /etc/default/foo
Please refer to Debian Policy Manual section 9.3.2 for details.