Rule source on GitHub

DowngradeStandaloneNullTrueFalseReturnTypeRector

Downgrade standalone return null, true, or false

 final class SomeClass
 {
-    public function run(): null
+    public function run(): mixed
     {
         return null;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp82\Rector\FunctionLike\DowngradeStandaloneNullTrueFalseReturnTypeRector;

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