ArcGIS Online and Desktop can display credits from the item description of tile layers (usually basemaps) in order to comply with copyright requirements.
Sometimes further attribution may be necessary for feature layers, but this can't be drawn directly from the credits in the feature service item details. This has resulted in users resorting to creating small transparent tile layers for the sole purpose of adding credit attribution to a web map.
Could the following please be considered?
1. Allow users to optionally add credit informaiton from feature service item details - this could be pushed from the item details on the service and/or pulled from the web map configuration
2. Dynamically update the web map copyright attribution according to the feature data displayed in the current map extent
Please let me know if you need any further informaiton.