As the title says, you can show and hide the plot legend index display (it is an array after all) in Edit mode, but there is no property to do that programmatically.
Since the NXG style plot legend scrollbar is buggy (see this report and the linked thread), providing this index display as a backup for users to navigate the plot list in the absence of a mouse wheel, is not elegant has its visibility cannot be controlled (AFAIK).