A form that follows the chosen variation
On a sold out simple product the form sits right under the "Out of stock" text: an email field and a button, with the address filled in for logged in customers. On a variable product it appears when the shopper picks a sold out option, so the alert is for exactly the size or colour they wanted. If every option is sold out they can ask to be told when any of them is back. The form works with and without JavaScript, with a honeypot, a nonce and a per visitor rate limit, and you can move it to the end of the summary or place it anywhere with the [dbis_form] shortcode.
Alerts that send themselves
Whenever a product's stock status becomes available, whether you edited the product, ran a CSV import, restocked through the REST API, or an order was cancelled and the stock came back, the plugin queues a sending run. Stock is checked again right before the emails go out, so a restock entered by mistake and corrected straight away sends nothing; an optional delay adds extra margin. Big lists go out in background batches through WooCommerce's Action Scheduler, each person gets one email per request, and every alert carries a one-click remove link.
Double opt-in and consent, if you want them
Guests can be asked to click a confirmation email before they are on the list, and the form can carry a consent checkbox with your own wording and a link to your privacy policy. Switch guests off altogether and the form asks shoppers to log in. Products on backorder can count as available or as sold out, as you prefer.
A waiting list you can actually run
WooCommerce → Stock Alerts lists every request with its status (waiting, unconfirmed, notified, cancelled), search by email or product, filters, bulk cancel, reactivate and delete, a "send now" per request and a CSV export. The Most wanted tab ranks sold out products by the number of people waiting, with a Notify now button for anything already back in stock. The product editor shows how many are waiting, by variation, and an Add a subscriber form handles requests that arrive by phone.
Emails and privacy done the WooCommerce way
"Back in stock" and "Confirm back in stock alert" are standard WooCommerce emails: edit the subject, heading and extra text under WooCommerce → Settings → Emails, preview them, and override the templates in your theme. Customers see and remove their requests in a Stock alerts tab in My Account, the WordPress privacy tools export and erase requests by email address, and old sent alerts are tidied away after a number of days you choose.
