The tag is present in Lintian version 2.104.318. That is the most recent version we know about.
Some packaging files obtain different names when they are copied from source to installation packages. Debhelper sometimes adds *.Debian extensions to NEWS, README and TODO files. That can be confusing.
Debhelper's behavior also depends on the filename.
This source package contains a file that debhelper will not find. The file will not be included in your installation packages. Important information, such as incompatibilties on upgrades, may not reach your users.
Please rename the file as indicated.