Welcome to the Shopify Community! Great question. There is a topic in the Community that addresses this towards the end of the thread that I'll share for starters. Essentially, you can use a third party app, or hire a developer to achieve this through some custom coding. There's isn't a way to do this through the natural Shopify Admin.
Otherwise, you can always hire a Shopify Expert for custom coding options. Just so you know as well, I've submitted some feedback on your behalf regarding this feature request. I can certainly understand why it's important to let your customers know that you'll be restocking a product if it's sold out.
Let us know if you have any questions!
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