Rule source on GitHub

DowngradeFinalPropertyRector

Change final property to @final tag

 class SomeClass
 {
-    public final $foo = 'bar';
+    /**
+     * @final
+     */
+    public $foo = 'bar';
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp84\Rector\Class_\DowngradeFinalPropertyRector;

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