Rule source on GitHub

DowngradeMixedTypeTypedPropertyRector

Removes mixed type property type definition, adding @var annotations instead.

 class SomeClass
 {
-    private mixed $property;
+    /**
+     * @var mixed
+     */
+    private $property;
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\Property\DowngradeMixedTypeTypedPropertyRector;

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