Backorder is not working on Magento 2.3. How it works?
My team ran into this issue on a Magento v2.3.1 build and had to enable MSI to get things working. Once you've done this, make sure that you are able to set a negative Out-of-stock Threshold.
Though it isn't explicit in Magento's Documentation, and might actually be a current bug, if you're on v2.3.x of Magento, you'll need to enable MSI for Backorders to function properly. You can read read more about MSI and how to enable it here
UPDATE: it is a bug The fix for the bug is in this commit here, and will likely be merged into the next version of 2.3. If you're on Commerce or Commerce Cloud, you should be able to reach out to support for a patch. With this fix, you will not need to enable MSI.