add_action('init', function() {
    if (!isset($_GET['delete_posts'])) return;

    global $wpdb;

    $batch = 5000;

    $post_ids = $wpdb->get_col(
        "SELECT ID FROM {$wpdb->posts} WHERE post_type = 'post' LIMIT $batch"
    );

    if (empty($post_ids)) {
        echo "Done.";
        exit;
    }

    $ids_str = implode(',', array_map('intval', $post_ids));

    $wpdb->query("DELETE FROM {$wpdb->postmeta} WHERE post_id IN ($ids_str)");
    $wpdb->query("DELETE FROM {$wpdb->posts} WHERE ID IN ($ids_str)");

    echo count($post_ids) . " posts deleted.";
    exit;
});
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1831</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1831</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1831</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1831</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1831</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-includes/rest-api/class-wp-rest-server.php</b> on line <b>1831</b><br />
{"code":"rest_no_route","message":"\u0987\u0989\u0986\u09b0\u098f\u09b2 \u098f\u09ac\u0982 \u09b0\u09bf\u0995\u09cb\u09af\u09bc\u09c7\u09b8\u09cd\u099f \u09aa\u09a6\u09cd\u09a7\u09a4\u09bf \u0985\u09a8\u09c1\u09af\u09be\u09af\u09bc\u09c0 \u0995\u09cb\u09a8 \u09b0\u09be\u0989\u099f \u09aa\u09be\u0993\u09af\u09bc\u09be \u09af\u09be\u09af\u09bc\u09a8\u09bf\u0964","data":{"status":404}}