Is it possible to add or edit domains after a feature service is published in ArcGIS Online?

Answer

As of the June 2018 update to ArcGIS Online, domains can be added and updated directly in ArcGIS Online through the Data tab in the hosted feature layer's item page. Refer to ArcGIS Online: Define attribute lists and ranges for more information.

An alternative is to create a local copy of the existing feature service in ArcMap, add or modify the domains in the local copy, and use the local copy to overwrite the existing feature service by navigating to File > Share As > Service > Overwrite an existing service.