WordPress get tags in “publish_post” hook

if you want to get all selected tag in “publish_post” try below code. it will give you selected tag from post. remove wp_die("all Tags"); after verify that you can get proper tags on publish post.

add_action( 'publish_post', 'post_published_notification', 10, 2 );
function post_published_notification( $ID, $post ) 
{
    $all_tags = $_POST['tax_input']['post_tag'];
    if ( $all_tags ) 
    {
        echo "<pre>";
        print_r($all_tags);
        echo "</pre>";
        wp_die("all Tags");
    }
}

It will output all selected tags by comma separated like :

new10,new4,new5,new6,new7,new8,new9

Let me know if this works for you!