The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
chown user.group is called in one of the maintainer
scripts. The correct syntax is
chown user:group. Using "." as a
separator is still supported by the GNU tools, but it will fail as soon
as a system uses the "." in user or group names.
Please refer to the chown(1) manual page for details.