The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The Document field in doc-base control file must be located at first line of the file. While unregistering documents, doc-base 0.8 and later parses only the first line of the control file for performance reasons.
Please refer to Debian doc-base Manual section 184.108.40.206 for details.