The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The architecture string in the source relation includes an unknown
architecture. This may be a typo, or it may be an architecture that dpkg
doesn't know about yet. A common problem is incorrectly separating
architectures with a comma, such as
[i386, m68k]. Architectures
are separated by spaces; this should instead be
Please refer to Debian Policy Manual section 5.6.8 for details.