The tag is present in Lintian version 2.104.232. That is the most recent version we know about.
The conffiles control file lists this path, but the path does not appear to exist in the package. Lintian may also emit this tag when the file exists, but the canonical name is used in the "conffiles" control file (e.g. if a parent segment are symlinks).
Note that dpkg and Lintian strips all whitespace from the right hand side of each line. Thus it is not possible for a file ending with trailing whitespace to be marked as a conffile.