Rule source on GitHub

DowngradeObjectTypeDeclarationRector

Remove the "object" param and return type, add a @param and @return tags instead

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

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp72\Rector\FunctionLike\DowngradeObjectTypeDeclarationRector;

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