Google Merchant Center updated its landing page requirements to say that products that are out of stock, those pages must have a grayed out add to cart/buy button on that page.
The document says, "For “out of stock” items, the “Buy” button must be greyed out on your product landing page." Here is that section:
Make sure availability matches your product data, and that products can successfully be added to a cart and delivered to a customer. For example, if a product is out of stock on your landing page, then update the availability [availability] attribute in your product data to match. That way, users won't try to purchase a product that you don't have in stock. For “out of stock” items, the “Buy” button must be greyed out on your product landing page.
This change was spotted by Emmanuel Flossie who wrote, "Hiding the button is not allowed. Leaving it fully clickable is not allowed. What you need to do now is disable the button."
Here is his video explaining this:
Forum discussion at LinkedIn.
