Delete taxonomy and delete all post related it

First of all we need to find the posts related to our taxonomy term. Then we will delete them one by one by running a foreach loop. Try below code. I hope that will work for you-

add_action('pre_delete_term', 'the_dramatist_delete_posts_on_taxonomy_delete', 1, 1 );
function the_dramatist_delete_posts_on_taxonomy_delete( $term ) {
    $args = array(
        'post_type' => 'post', // post_type
        'tax_query' => array(
            array(
                'taxonomy' => 'post_tag', // taxonomy_name
                'field' => 'id',
                'terms' => $term
            )
        )
    );

    $posts = get_posts($args);

    foreach ( $posts as $post ){
        wp_delete_post( $post->ID, true );
    }
}