Toot by Lanie CarmeloLanie Carmelo (caneandable.social)

If the image is a true featured image, as in, for example, you're adding it to the page via the featured image box in a WordPress installation, it doesn't require alt text. And in the case of WordPress, even if you add alt text it won't be displayed because you're essentially dealing with a decorative image. If you still want to add alt text, something like "Me holding my ten-year-old miniature pincher squeaker".

Toot by Lanie CarmeloLanie Carmelo (caneandable.social)

It'll ask, because if that image is ever used anywhere else on the site, the alt text comes with it. But as far as the featured image itself is concerned I can't think of a theme that will actually display alt text with a featured image since those are decorative, and decorative images that don't communicate anything don't require alt text.