A Silverstripe DataExtension which adds subsite support to the redirectedurls module.

extensions.yml

SilverStripe\RedirectedURLs\Admin\RedirectedURLAdmin:
  extensions:
    - SilverStripe\Subsites\Extensions\SubsiteMenuExtension

SilverStripe\RedirectedURLs\Model\RedirectedURL:
  extensions:
    - SubsiteAwareRedirectExtension

SubsiteAwareRedirectExtension.php

<?php

use SilverStripe\Forms\FieldList;
use SilverStripe\ORM\DataExtension;
use SilverStripe\ORM\DataQuery;
use SilverStripe\ORM\Queries\SQLSelect;
use SilverStripe\Subsites\Model\Subsite;
use SilverStripe\Subsites\State\SubsiteState;

class SubsiteAwareRedirectExtension extends DataExtension
{
    private static $has_one = [
        'Subsite' => Subsite::class,
    ];

    public function onBeforeWrite()
    {
        if (!$this->owner->SubsiteID) {
            $this->owner->SubsiteID = SubsiteState::singleton()->getSubsiteId();
        }
    }
  
    /**
     * Update any requests to limit the results to the current site
     * @param SQLSelect $query
     * @param DataQuery|null $dataQuery
     */
    public function augmentSQL(SQLSelect $query, DataQuery $dataQuery = null)
    {
        if (Subsite::$disable_subsite_filter) {
            return;
        }
      
        if ($dataQuery && $dataQuery->getQueryParam('Subsite.filter') === false) {
            return;
        }

        $subsiteID = SubsiteState::singleton()->getSubsiteId();

        if ($subsiteID === null) {
            return;
        }

        // The foreach is an ugly way of getting the first key :-)
        foreach ($query->getFrom() as $tableName => $info) {
            $where = "\"$tableName\".\"SubsiteID\" IN ($subsiteID)";
            $query->addWhere($where);
            break;
        }

        $sect = array_values($query->getSelect() ?? []);
        $isCounting = strpos($sect[0] ?? '', 'COUNT') !== false;

        // Ordering when deleting or counting doesn't apply
        if (!$isCounting) {
            $query->addOrderBy('"SubsiteID"');
        }
    }


    public function updateCMSFields(FieldList $fields)
    {
        $fields->removeByName('SubsiteID');
    }
}