Why does Schema Pro no longer say my posts need an image?

Because Google does not require one for an article. Google's article documentation lists the image, headline, author and dates as recommended, with no required properties, so a post without a featured image is not a page to fix. A property is now marked required only where Google's documentation for that type says so: a recipe's or a movie's image, a product's image and price, for example. The full list is in the documentation under Validation.