Automatically add featured image Alt Text from post title

The following code snippet will automatically generate Alt Text for all featured images from the Post Title.

function featured_image_alt_text($metadata, $object_id, $meta_key, $single) {
    if(isset($meta_key) && $meta_key == '_wp_attachment_image_alt'
        && get_post_thumbnail_id() == $object_id
        && $single === true
    ){
        $original_value = get_post_meta(
            $object_id, '_wp_attachment_image_alt', false
        );
        if(empty($original_value)){
            return get_the_title();
        }
    }
    return $metadata;
}

add_filter('get_post_metadata', 'featured_image_alt_text', 10, 4);

If you are looking for more settings, check the SEO Friendly Images plugin.