When pinned products are removed from the Category or Collection (changes in the condition or removed manually), the message 'Product not available in Category (or) Collection' is displayed. These products will not be available for merchandising in the Category or Collection.
The below image is an example from a Magento store where the 5 skirts were pinned atop of the category when these products were in stock or included in the category. While products were pinned, they could have been removed from the category. Hence, to inform the merchandiser who pinned these products for business reasons, the message 'Product not available in Category', is displayed so they can take appropriate actions if required.