By default, Arrives By will not display when products/variants are out of stock.
You can add a custom delivery estimate for these out of stock products as long as they meet the following conditions:
The product/variant must have "Track quantity" enabled in its Inventory settings.
The product/variant must have "Continue selling when out of stock" enabled.
An "out of stock processing time" is set in the Arrives By app settings.
Arrives by will use this out of stock processing time as well as the transit time for that product to show a delivery estimate.