Looks like the warning is gone from our Data Management page
Now we have another:
Warning: This Dataset comes from an Item that points to more than one layer. For Open Data, we recommend that Items reference individual layers.
and one I didn't catch yesterday:
Warning: messages.inspectors.display_field.not_present
All of our open data layers are referenced as individual feature layers so I'm not sure what's generating that warning.
I think the warning found in our CIty Boundary layer is because there are only "shape" and "objectID" fields visible.