If you enabled inventory for a product, it will be automatically marked as sold out when the available stock becomes zero. Out of stock products will be displayed under the "Sold out" section. The description for this section cannot be changed.

Thanks. I’d like to suggest a future tweak to make the message customizable.

Customizable generally, not to each product. we do a once a year fundraiser so the message that is currently there doesn’t really fit our business.