Thanks for bringing your question here to the Community forum. It sounds like you're hoping to continue selling products that are sold out. That's a great way to ensure that you don't miss out on sales during periods of low inventory. I'm here to help!
I'm happy to make some app suggestions for you. Before we get into that, I also want to mention that you have the option to do this through Shopify. This is done by creating an alternate product template, and editing your payment settings. You can also use this customization to continue selling products that are on backorder. Keep in mind, this is an advanced tutorial which means it is not supported by Shopify. However, if you feel comfortable with coding this is a great solution. Alternatively, you could hire a Shopify Expert to help you implement this.
If you're going to edit the code of your theme, I highly recommend duplicating your theme first, and learning how to roll back to an older version of your theme here. That way, you can reverse the changes if needed. If you prefer, there are also some apps that can help achieve this, such as:
Most apps offer a free trial so that you can test it out before committing. If you decide to not use the app, just ensure that you uninstall it before the trial ends to avoid getting charged. You can learn more about this process in our help document on Working with apps.
Please let me know if you have any questions about that. In the meantime, how has everything been going with your business? If there is anything else I can help with, let me know!
Just checking in with you to make sure you receive all the assistance you need. If there's anything else that I can help with, please let me know.
We understand it may be a challenging journey to find the right app for your specific functionality. Having read through your needs, I believe that our app Back in Stock - Restock Rocket provides the functionality you are looking for, for FREE. Our app inserts an "Email me when Available" button on the product pages that have out-of-stock products (i.e. inventory qty as 0). You will be able to see the requests for those products from our App and once you have restocked them from the admin dashboard, our app automatically sends a back-in-stock email alert to customers that have requested that product.
We know this may not exactly resolve your issue for Pre-Order but it is a good workaround that is used by major e-commerce stores. We provide 24x7 support and our app is very easy to use. We would love for you to try out our app and provide us with feedback.
Restock Rocket Family