How to truncate alt text for featured images on content archives in Genesis. https://sridharkatakam.com/how-to-truncate-alt-text-for-featured-images-on-content-archives-in-genesis/

functions.php

// Function to truncate post titles
function customTitle( $limit ) {

    $title = get_the_title( $post->ID );

	if ( strlen( $title ) > $limit ) {
      $title = substr( $title, 0, $limit ) . '...';
    }

    return $title;
}

// genesis_entry_header action hook is used here because it is the one immediately above genesis_entry_content. Remember: Hook as late as possible.
add_action( 'genesis_entry_header', 'sk_custom_post_image' );
function sk_custom_post_image() {

	// if we are on a content archive page and featured images are set to be shown on content archives in Genesis theme settings
	if ( ! is_singular() && genesis_get_option( 'content_archive_thumbnail' ) ) {

		// remove the standard featured image output by Genesis per theme settings
		remove_action( 'genesis_entry_content', 'genesis_do_post_image', 8 );

		// add custom featured image output
		add_action( 'genesis_entry_content', 'sk_do_post_image', 8 );

	}

}

// Function to set the alt text of featured image to truncated post title text
function sk_do_post_image() {

	$img = genesis_get_image( array(
		'format'  => 'html',
		'size'    => genesis_get_option( 'image_size' ),
		'context' => 'archive',
		'attr'    => genesis_parse_attr( 'entry-image', array ( 'alt' => customTitle( 20 ) ) ),
	) );

	if ( ! empty( $img ) ) {
		printf( '<a href="%s" aria-hidden="true">%s</a>', get_permalink(), $img );
	}

}