Use Captions as alt text.

iap-use-captions-as-alt-text

/**
 * Use Captions as alt text.
 *
 * @param $attributes 		(array) Associative array of image attributes.
 * @param $image_id 		(int) ID of the current image.
 * @param $parent_post_id	(int) ID of the post the image is inserted into. 0 for images not attached to a post.
 * 
 * @author Arun Basil Lal
 * @link https://imageattributespro.com/codex/iaffpro_image_attributes/
 */
 function prefix_use_captions_as_alt_text( $attributes, $image_id, $parent_post_id ) {

	$image = get_post( $image_id );
	
	// Image caption is saved as post_excerpt.
	if ( ! empty( $image->post_excerpt ) ) {
		$attributes['alt_text'] = $image->post_excerpt;
	}
	
	return $attributes;
}
add_filter( 'iaffpro_image_attributes', 'prefix_use_captions_as_alt_text', 10, 3 );