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":"\u092f\u0942\u0906\u0930\u090f\u0932 \u0914\u0930 \u0928\u093f\u0935\u0947\u0926\u093f\u0924 \u092a\u094d\u0930\u0923\u093e\u0932\u0940 \u0938\u0947 \u092e\u093f\u0932\u0924\u093e \u0915\u094b\u0908 \u092e\u093e\u0930\u094d\u0917 \u0928\u0939\u0940\u0902 \u092e\u093f\u0932 \u0938\u0915\u093e\u0964","data":{"status":404}}