Rule source on GitHub

DowngradePureIntersectionTypeRector

Remove the intersection type params and returns, add @param/@return tags instead

-function someFunction(): Foo&Bar
+/**
+ * @return Foo&Bar
+ */
+function someFunction()
 {
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\FunctionLike\DowngradePureIntersectionTypeRector;

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