Answer
ROUTING_OPERATION_API.Get_Note_Text exists to do exactly this, but only from IFSAPP10 onward — that specific function doesn't exist yet on IFSAPP9. On IFS9, retrieve the note text directly from the SHOP_ORDER_OPERATION_JOIN view or the ROUTING_OPERATION_TAB table instead of trying to call an API function for it. If you need this available as a reusable function rather than an inline query, you can write a small custom utility package function modeled on the existing Get_Note_Id pattern to wrap that same lookup.