Writing from a Wikimedia Commons perspective, could someone confirm that the use of "metadata" in this RfC is meant as structured data or data from Wikidata, not the "metadata" that exists for files on the database which is extracted from EXIF data?
Topic on Talk:Requests for comment/Schema update for multiple content objects per revision (MCR) in XML dumps
Technically it refers to any content stored in Requests for comment/Multi-Content Revisions's slots. Such data can be generated by an extension such as Wikibase or even mediawiki itself. In terms of commons, it is currently obtaining data from Wikidata and from wikibase installed in commons itself.
Media's metadata like Exif doesn't seem to get dumped at all(https://dumps.wikimedia.org).
Although technically, it can probably be found in elasticsearch dumps, but that's unrelated to this RFC.