AGOL says that "cluster_count" is an available field in clustered layers. What are the names of the other fields used by the default clustering popup?
I would like to customize the popup and reformat the existing popup.
I know that in 4.*, they are:
But the last 2 fields don't work in 3.x.
As far as I know these there are no more than the ones you listed. It only depends on the renderer which ones are available and which are not. For example with uniqueValueRenderer there is only count available and the type gives you the most common type of the features which are clustered together.
See Styles and Configuration: