The tag is present in Lintian version 2.104.244. That is the most recent version we know about.
All configuration files must reside in
/etc. Files below
/usr may not be marked as conffiles since
/usr might be
mounted read-only. The local system administrator would therefore not
have a chance to modify this configuration file.
Refer to Debian Policy Manual section 10.7.2 for details.