The correct way is that calling the Resouce model Magento\Newsletter\Model\ResourceModel\Subscriber directly at your model class or service contact class instead of call Model class getResource().
As Magento following Service concept design pattern https://devdocs.magento.com/guides/v2.4/extension-dev-guide/service-contracts/service-contracts.html, So model class used as Data provide a class that getResource() is deprecated at magento2.
Like \Magento\Cms\Api\Data\BlockInterface is rewrite by \Magento\Cms\Model\Block.
/**
* @var $subscriber \Magento\Newsletter\Model\ResourceModel\Subscriber
*/
$subscriberResource = $this->subscriberResource;
$array = $subscriberResource->loadByCustomerData($customer);
Check the How to implement service contract for a custom module in Magento 2? about service contract.
Example:
<?php
namespace {NameSpace};
// Alias of Class
use Magento\Newsletter\Model\ResourceModel\Subscriber as SubscriberResourceModel;
class {Classname}{
private $subscriberResource;
public function __construct(
SubscriberResourceModel $subscriberResource,
.....
) {
.....
$this->subscriberResource = $subscriberResource;
....
}
}