Rule source on GitHub

DowngradeReflectionPropertyGetDefaultValueRector

Downgrade ReflectionProperty->getDefaultValue()

 class SomeClass
 {
     public function run(ReflectionProperty $reflectionProperty)
     {
-        return $reflectionProperty->getDefaultValue();
+        return $reflectionProperty->getDeclaringClass()->getDefaultProperties()[$reflectionProperty->getName()] ?? null;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\MethodCall\DowngradeReflectionPropertyGetDefaultValueRector;

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