The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
The specified series file for patches is vendor-specific.
Source packages may contain vendor (i.e. distribution) specific patches, but such packages must not be uploaded to the Debian archive if they are used in conjunction with vendor-specific series files.
Vendor specific series files were carefully implemented as a
feature. Unfortunately, they currently conflict with some workflow goals
in Debian. They are presently disallowed in Debian.
The preferred approach for distributions other than Debian is now to
apply such patches programmatically via
debian/rules. You can
also create multiple, vendor-specific sources and upload them separately
for each distribution.
The decision to prohibit the use of that particular
in Debian was made by the project's technical committee in consideration of
other planned workflow modifications, and may be revisited.
You should only see this tag in Debian or other distributions when targeting an upload for Debian.
Refer to Bug#904302, Bug#922531, and https://lists.debian.org/debian-devel-announce/2018/11/msg00004.html for details.