I'll chime in here because I'm looking for the same data point. To clarify, I'm looking for a way to share what we System Admins can see in that 'Interface>Update Feeds' with our stakeholders. When we were maturing in Workfront, we didn't realize there was a hard limit to how many fields we could track in Update Feeds, so we just willy-nilly and rather randomly enabled this for fields as we built out our workflows. So we reached our limit. A ticket into Support yielded positive results in that they were able to bump our limit up from 300 tracked fields to 500. So that was glorious; but the underlying matter still exists. Pressing Support to help out on a report for this yielded a response paraphrased as: This solution will require Professional Services, so you'll have to pay additional money for more help on this.
There must be a way to show my stakeholders all of the custom fields that are currently being tracked so they can sift through them and determine if that tracking is neccessary. I tried digging through the API explorer but couldn't find anything that would reference that information. I agree with @Stacey_Robertson - there *should be* a isJournaled reference point (like there's an 'isRequired', for example), but there isn't. I wonder why not.
A Kick-Start export of Custom Data didn't get me anything. Reporting seems to be a dead-end, and there doesn't seem to be a way to export the date from the 'Setup>Interface>Update Feeds>Tracked Fields>Custom Fields' page.
So if anyone out there has made any headway on this, tag me and let me how you did it!!