The Thumbnail aspect Ratio Issue

If you want to remove the height value from your img URL, you can use this function:

add_filter( 'post_thumbnail_html', 'remove_thumbnail_height', 10, 5 );
function remove_thumbnail_height( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
    $html = preg_replace( '/height=\"\d*\"https://wordpress.stackexchange.com/", "", $html );
    return $html;
}

This will replace the height with an empty value. Note that you can’t use Auto as a value for the height property. It won’t be validated by w3 validator. However, you can set the height to auto in your CSS:

.wp-post-image {
    height: auto;
}