Currently the Metadata standard ISO 19139 from 2007 is supported in ArcGIS Online / Enterprise. This standard has been revised by ISO 19139-1 and has the status 'Withdrawal' (also see https://www.iso.org/standard/32557.html). To keep metadata in line with the newest developments we would also like to have support for the new metadata profile ISO 19139-1 (https://www.iso.org/standard/67253.html).
