Rule source on GitHub

DowngradePropertyPromotionRector

Change constructor property promotion to property assign

 class SomeClass
 {
-    public function __construct(public float $value = 0.0)
+    public float $value;
+
+    public function __construct(float $value = 0.0)
     {
+        $this->value = $value;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\Class_\DowngradePropertyPromotionRector;

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