In stock notification functionality for sold out, backordered and preordered products are managed by 3 separate boolean options in the Order Management System (OMS) at (Settings / Boolean Options), including:
In Stock Notification (Sold Out)
In Stock Notification (Backordered)
In Stock Notification (Preordered)
For each of these boolean options, when they are on, customers can click a link in your online store to request notification for a product or SKU which displays as sold out, on backorder, or preordered, respectively. Once you add inventory to the product in your OMS, email notifications are sent by the system automatically.
To see who has requested In Stock notifications, navigate in your OMS to Products / Export Traffic Tools / Instock Notification / Excel.
The ? tooltips in the OMS near each boolean option and the instock notification export provide complete details.