The tag is present in Lintian version 2.104.244. That is the most recent version we know about.
The "isdefault" flag on debconf questions is deprecated as of debconf 0.5.00, and has been replaced by "seen" with the inverse meaning. From debconf 0.5 onwards there should be very few reasons to use isdefault/seen anyway, as backing up works much better now. See /usr/share/doc/debconf-doc/changelog.gz for more information.
The misuse of isdefault often leads to questions being asked twice in one installation run, or, worse, on every upgrade. Please test your package carefully to make sure this does not happen.