Use the Products portion of the Theme Settings to automatically select the first available variant or prompt customers to choose a variant (no default selection), then select a sold-out display option for your variants.
Automatically select the first available variant
By default, the Auto-select first available variant setting is enabled. This allows customers to either add the default selection or select another variant before adding it to their cart.
Prompt customers to choose a variant (no default selection)
Click to disable the Auto-select first available variant setting to have no default selection. Instead, customers are prompted to choose from the available options before adding the variant to their cart.
Select a sold-out display option
When variants are sold out/out of stock, they can be concealed, visible but disabled, or visible and selectable.
Hidden
Use the Hidden option to conceal out-of-stock variants.
Disabled
Use the Disabled option to keep out-of-stock variants visible but not selectable by customers.
Selectable
Use the Selectable option to keep out-of-stock variants visible and selectable by customers. While the variants are selectable with this option, they cannot be added to the cart.
This feature is often a requirement for 3rd-party apps for back-in-stock notifications.