Saving the DWG/DXF Export Setup Settings to a text file is no longer an option in Revit, the settings are retained/saved in the Revit project file.
If you need to copy these settings between projects, you can use “Transfer Project Standards”

You need to have both the Source (with the export settings defined) and Target (where you want to copy the settings to) projects open in Revit…

1. In the Target project, select “Transfer Project Standards” from the Ribbon... (Settings panel of the Manage tab)
   
   
   
2. The source project name should be listed in the “Copy from:” drop-down (this will list multiple projects if you have other projects open)
   In the “Select Items To Copy” dialog, check “DWG/DXF Export Setup Settings”
   
   
   
3. Click OK, done