Has anyone else seen this Datum transformation error from NAD 83 to NAD 1983 since updating Pro to v. 2.8? There are no other choices in the dropdown box other than none and you can't get any information with the Details selection either? I suspect this may have something to do with having some Canadian data layers in my project but not sure. I can't seem to find a layer in my project that actually says it's using NAD 83 so I'm stumped.
Isn't NAD 83 same as NAD 1983?
If the data (with NAD 83) is coming from another application, you could use Define Projection to set it to NAD 1983 (GCS or PCS as required).
If there is no existing layer with NAD 83, you don't have to worry about the warning. Maybe a layer was added and then removed from the Content.
If it still concerns you, create a new project > copy/paste the existing layers from the current project to the new project. You shouldn't be seeing the third transformation option there.
I received the same message in one of my projects after I upgraded to 2.8. All of the layers are in Southern California, and I received a Transformation Warning Notification for the one map in my project that has some layers projecting on the fly.