Rule source on GitHub

DowngradeArrayIsListRector

Replace array_is_list() function

-array_is_list([1 => 'apple', 'orange']);
+$arrayIsList = function (array $array) : bool {
+    if (function_exists('array_is_list')) {
+        return array_is_list($array);
+    }
+
+    if ($array === []) {
+        return true;
+    }
+
+    $current_key = 0;
+    foreach ($array as $key => $noop) {
+        if ($key !== $current_key) {
+            return false;
+        }
+        ++$current_key;
+    }
+
+    return true;
+};
+$arrayIsList([1 => 'apple', 'orange']);

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\FuncCall\DowngradeArrayIsListRector;

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