Currently in doc-base-generated listings, nothing tells whether an
entry refers to a non-free software item.  It would be useful, and
would support our social contract, to allow for some means of tagging
such documents, so that people using a Debian system on which non-free
software is installed do not mistake a non-free package from a
dfsg-free one.

Not sure how to do this reliably, however.  Requiring cooperation from
non-free packages clearly will not work for 3rd-party software.  Maybe
this could be determined for Debian packages by the dist in which the
package is located (main/non-free), and non-Debian packages (as
defined by the Origin: package field ?) would get an "undetermined
licencing" mention instead of "non-free software", unless they
explicitely specify dfsg or non-dfsg in the doc-base spec.