Hello,

This is not included in our current planning. However, you can follow this item to get notified about changes in its status.

In the meantime, you can achieve your requirement if you edit the ControlTemplate of the RadRibbonBackstage control. Based on the theme and the BackstagePosition property, the template name vary between "TabControlTemplate2013" and "TabControlTemplate2010". The template targets RadTabControl, but it is applied to RadRibbonBackstage (which derives from RadTabControl). In the template, remove the RadRibbonButton with Style set to BackstageCloseButtonStyle or change its Visibility. Then apply the customized template via the Template property of RadRibbonBackstage. 

An alternative approach is to use the visual tree helper methods to get the close button in code and hide it there. For example:

private void RadRibbonBackstage_Loaded(object sender, RoutedEventArgs e)
{
	var backstage = (RadRibbonBackstage)sender;
	var ribbonButton = backstage.ChildrenOfType<RadRibbonButton>().FirstOrDefault(x => x.Command == RibbonCommands.CloseBackstage);
	if (ribbonButton != null)
	{
		ribbonButton.Visibility = Visibility.Collapsed;
		// Alternatively, you can data bind bind the Visibility to a property 
		// or save the button in a field and update it on an event in code-behind
	}
}

I hope that helps.

Regards,
Martin Ivanov
Progress Telerik