The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The Files paragraph uses a reference to a license which is only defined in the Header paragraph. The copyright specification requires that the Files paragraph either contains the full license itself or references a "stand-alone" License paragraph, and not the Header paragraph.
Please refer to https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/ for details.