Rule source on GitHub

DowngradeIsEnumRector

Downgrades isEnum() on class reflection

 class SomeClass
 {
     public function run(ReflectionClass $reflectionClass)
     {
-        return $reflectionClass->isEnum();
+        return method_exists($reflectionClass, 'isEnum') ? $reflectionClass->isEnum() : false;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\MethodCall\DowngradeIsEnumRector;

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