Rule source on GitHub

DowngradeInstanceofStringableRector

change instanceof Stringable to method_exists

-$obj instanceof \Stringable;
+is_object($obj) && method_exists($obj, '__toString');

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\Instanceof_\DowngradeInstanceofStringableRector;

return RectorConfig::configure()
    ->withRules([
        DowngradeInstanceofStringableRector::class,
    ]);