Hi everybody,
If a layer doesn't have a defined coordinate system, it's not projected on-the-fly. It can't be. The software doesn't know anything about it--the units, the map projection, etc. It will be displayed as-is, just as it is in ArcMap. You can still investigate the unknown data's coordinates by either zooming to it and looking at the status bar or opening its properties and looking at its extent.
To match the "clear" option in ArcMap in ArcPro:
1. Add the unknown data to a map
2. Add reference data that you want to compare it to. This can be a base map.
3. Make sure the map's coordinate system is set to the reference data's coordinate system.
4. So the reference data is displayed where it is natively. Nothing is happening to its coordinates or extent.
5. The unknown data is also being displayed natively. Nothing is happening to its coordinates or extent.
Now you can compare the location of the unknown data against the reference data.
We've reviewed the decision to not let ArcGIS Pro have an "unknown" xy coordinate system a few times. We've always made the same decision. There are too many side issues that arise if we don't have a known coordinate system.
Melita