Doesn't look like Exportable is available in GridColumnSettings. I'm using the LoadSettings feature on a Grid and I want to set some columns to not be exported to Excel. Exportable is a setting when using the column binding but I don't see it in the GridColumnSettings class.
If the application is using an older version of the Telerik UI, you should either update the version of the product or try to implement some custom functionality that prevents the column from being included in the exported file. My recommendation is to update the version of the product. For more information, please review the following article: https://docs.telerik.com/aspnet-mvc/installation-mvc/upgrading/upgrade