I think it would be an incredibly handy feature if you could reset a layer to its defaults in the service definition. We recently had an issue where an Arcade display expression for one layer wasn't set up correctly and had to be adjusted. No sweat just fix it and overwirte the layer or use the ArcGIS Online assistance and reload the layers JSON. Now the problem we have is the 20 or so maps that have that layer in it now also need to be updated to the new arcade expression. Very time consuming and painful to do.
I would love to see a reset layer to defaults button. These would be the defualts in the service definition.
Additionally it would be great if you had the options for a full reset or refresh all values that haven't been explicity changed in the map.
I have maps where the same layers are used for different purposes and most of the settings are default but with a different label expresion or symbology. It's currently a nightmare if the client update their data structure the only effective way is to delete your entire map library and start again.
I also think there is merit in having the map layers reference the definitions in the service layer directly unless they have been set specifically in the map. I wouldn't have this issue if this was the case as all of the maps would have been updated when the service was updated.