The package sgml-data includes non-Free files in the sgml/entities/sgml-iso-entities*/ directories and the sgml/html/entities/ directory under the following licence:
'Permission to copy in any form is granted for use with conforming SGML systems and applications as defined in ISO 8879, provided this notice is included in all copies.' Unless 'conforming SGML systems and applications' is broadly defined in ISO 8879, then redistribution is highly restricted (I cannot check ISO 8879, because it seems there is no gratis copy). Files under the same licence are also included in linuxdoc-tools in the directories iso-entities/entities/ and entity-map/sdata/ (for example ISOtech). Furthermore, files in the directory classpath/tools/resource/gnu/classpath/tools/gjdoc/dtd/ent/ in GCC are based on these files. The SGML entity files are needed to build documentation for several projects. It seems like it would be difficult to write the SGML entity files much differently, so it is possible that they are not copyrightable.