Will the module work with the Magento MSI functionality disabled?
Starting from version 2.1.2, the dependency on Magento MSI was removed from a basic build and moved to a separate module component. The extension works on single-source installations out of the box, and the compatibility with MSI is added with an optional system package: amasty/custom-stock-status-msi.
If you don't use the Magento MSI functionality, simply install only the main Custom Stock Status extension (without amasty/custom-stock-status-msi). It will work with a single-source store out of the box.