The tag is present in Lintian version 2.109.2. That is the most recent version we know about.
The DEP 12 metadata file is not well formed. The document level must be a YAML mapping:
Some-Field: some-value Another-Field: another-value
Sometimes, the fields are mistakenly prefaced with a hyphen, which makes them a YAML sequence. In that case, please remove the hyphens.
Please refer to https://dep-team.pages.debian.net/deps/dep12/ for details.