Rule source on GitHub

DowngradeStaticTypeDeclarationRector

Remove "static" return and param type, add a "@param $this" and "@return $this" tag instead

 class SomeClass
 {
-    public function getStatic(): static
+    /**
+     * @return static
+     */
+    public function getStatic()
     {
         return new static();
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\ClassMethod\DowngradeStaticTypeDeclarationRector;

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