The availability of your products can be shown on the website to reassure your customers.
To display this, open the Sales tab in the product detail form and select an option in Availability.
A custom warning message can be anything related to a stock out, delivery delay, etc.
This tool does not require the Inventory app to be installed.
If one item is no longer sellable, unpublish it from your website. If it comes to one particular product variant, deactivate the variant in the backend (see How to adapt the prices to my website visitors).