The property was only introduced to support reading data from old Magnolia.

In new Magnolia, the usage of this property should be clearly marked as deprecated. At https://docs.magnolia-cms.com/product-docs/Releases/Release-notes-for-Magnolia-CMS-6.2.1.html#_new_supporti18n_property_in_jcrchildnodeprovider, add the deprecation note retroactively. Otherwise, the release note sounds as if this was a new feature that we're okay with customers using (though we don't have documentation for it).