Allow editing of Attributes in joined fields.
Use Case:
[Feature Class -> Relationship Class -> Table ] published to ArcGIS Online as Feature Service
Use Join to style the Feature Class based on a column in the Table and use this directly for editing.
Updating fields in Joined table using ArcGIS Pro - Esri Community
I know there are workarounds, but they are not feasible with specific use cases.
For example ArcGIS Online does not support Attribute Rules which could be used to update a related table.