From the 2019 UC Q&A - reposting here for wider distribution
Standards-compliant metadata is already supported for items throughout the ArcGIS platform for viewing and editing in ArcMap, ArcGIS Pro, ArcGIS Online, and ArcGIS Enterprise. Support for more recent updates to the ISO 19115 standard (-1/2/3) are under development.
ArcGIS Online and ArcGIS Enterprise now allow users to view layer-level metadata for hosted feature layers from the layer's item page. The metadata is displayed in the style configured by the organization. Additionally, when exporting hosted feature layers to a file geodatabase or to shapefiles, layer-level metadata is included in the exported feature classes. At present, the layer-level metadata is read-only; it will be editable in the future.
In ArcGIS Pro 2.1, we introduced the ability for map layers either to reference a read-only version of their source data's metadata (the default), or to have their own metadata independent of the source data’s metadata. With the release of ArcGIS Pro 2.2, users are able to import metadata to an item, save a copy of ArcGIS metadata to a file on disk, and export an item’s metadata to a standard-format XML file. Users are also able to upgrade metadata created in ArcGIS Desktop 9.x (for example) to the ArcGIS metadata format so it can be viewed and managed in ArcGIS Pro. When publishing a map from ArcGIS Pro, the map’s full ArcGIS metadata will be published with the data and transferred to the portal item. Using ArcGIS Pro’s metadata import capabilities full metadata can be transferred from local maps to the portal items that were published with earlier versions of ArcGIS Pro.
When ArcGIS Pro 2.2 is used together with ArcGIS Enterprise the process of updating metadata for published content will be more streamlined than ever. If you publish a map service where layers reference data in an enterprise geodatabase that is registered as a data store item, and the map layers reference the data source’s metadata—updates to the feature class metadata are instantly available in the portal when someone views metadata for the service’s layers.
With the Pro 2.3 release (Jan ‘19), you can create Metadata for a Project and extend Pro with the Pro Metadata Toolkit (https://github.com/Esri/arcgis-pro-metadata-toolkit). The ArcGIS Pro Metadata Toolkit provides resources to create an Add-In that allows you to customize the pages that appear in the ArcGIS Pro metadata editor to suit your organizational, community, or regional requirements.
For the upcoming Pro 2.4 release, Esri will begin a phased approach for supporting the newer ISO 19115-1 and the 19115-3 specifications by first allowing import of an ISO 19115-3 metadata file; however, this file will lose any new attributes not supported by old standard upon import. Full support for ISO 19115-1/2/3 is planned for a Pro 2.5 release.