./composer.json is valid for simple usage with composer but has strict errors that make it unable to be published as a package: See https://getcomposer.org/doc/04-schema.md for details on the schema name : The property name is required description : The property description is required No license specified, it is recommended to do so. For closed-source software you may use "proprietary" as license.
Not sure we can do a license easily, but presumably we could do a name and description?
mediawiki/vendor is not meant to be a public package that would be submitted to Packagist so I don't think this is really a problem. I suppose it wouldn't hurt anything to add the missing metadata, but if so we should also be sure to change the type from the default of "library" to "project". For a license I think GPLv2+ should be fine. This work is a derivative of MediaWiki itself.
The fields were intentionally removed in rMWVD46f919a283f7: composer.json: Remove metadata fields so it can't be published as a package after someone published it as a package on packagist.