Rule source on GitHub

DowngradeMixedTypeDeclarationRector

Remove the "mixed" param and return type, add a @param and @return tag instead

 class SomeClass
 {
-    public function someFunction(mixed $anything): mixed
+    /**
+     * @param mixed $anything
+     * @return mixed
+     */
+    public function someFunction($anything)
     {
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\FunctionLike\DowngradeMixedTypeDeclarationRector;

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