Rule source on GitHub

DowngradeAbstractPrivateMethodInTraitRector

Remove "abstract" from private methods in traits and adds an empty function body

 trait SomeTrait
 {
-    abstract private function someAbstractPrivateFunction();
+    private function someAbstractPrivateFunction() {}
 }

Configure your rector.php:

<?php

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

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