Hello, @jamarzy .
Thank you for posting this question!
If you’d like to learn more about the features on Shopify Plus, then we’ve got a guide on that here: Shopify Plus. However, it would be an easier option to use a third-party back in stock app as they provide the tools to easily manage alerts and keep track of customers that have requested to be notified. The alternative will likely require a significant amount of custom coding and flows in order to replicate what these apps can do.
For example, Back in Stock: Restock Alerts is great for getting started as they offer a free plan that allows you to send up to 10 notifications. The app has built-in integrations with mailing services, such as Klaviyo, and the app developer offer 24/7 support. Should you need to get help with a third-party app, then the app developer is always available to assist you. Simply reach out to them using the contact information found at the bottom of their app’s page.
If you have any other questions on this topic, then don’t hesitate to let me know!