Magento 2.3.2 email sending code not working in Magento 2.3.3

You can remove array from addTo([$sendTo]) function. If you'll remove that and add only single email address there then it will work.

<?php

namespace Vendor\Modulename\Observer;

use Magento\Framework\Event\ObserverInterface;
use \Magento\Store\Model\StoreManagerInterface;
use \Magento\Framework\Mail\Template\TransportBuilder;
use \Magento\Framework\Translate\Inline\StateInterface;
use Psr\Log\LoggerInterface;

class SendMail implements ObserverInterface
{
    protected $scopeConfig;

    const XML_PATH_TEMPLATE_SUBJECT = 'mycustomdemo/general/template_subject';
    const XML_PATH_SENDER_NAME = 'mycustomdemo/general/sender_name';
    const XML_PATH_SENDER_EMAIL = 'mycustomdemo/general/sender_email';
    const XML_PATH_SENDER_MESSAGE = 'mycustomdemo/general/sender_message';

    public function __construct(
        StoreManagerInterface $storeManager,
        \Magento\Framework\Message\ManagerInterface $messageManager,
        TransportBuilder $transportBuilder,
        LoggerInterface $logLoggerInterface,
        StateInterface $inlineTranslation,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
    ) {
        $this->storeManager = $storeManager;
        $this->messageManager = $messageManager;
        $this->transportBuilder = $transportBuilder;
        $this->inlineTranslation = $inlineTranslation;
        $this->logLoggerInterface = $logLoggerInterface;
        $this->scopeConfig = $scopeConfig;
    }
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        try {
                $storeScope = \Magento\Store\Model\ScopeInterface::SCOPE_STORE;
                $templateSubject = $this->scopeConfig->getValue(self::XML_PATH_TEMPLATE_SUBJECT, $storeScope);
                $senderName = $this->scopeConfig->getValue(self::XML_PATH_SENDER_NAME, $storeScope);
                $senderEmail = $this->scopeConfig->getValue(self::XML_PATH_SENDER_EMAIL, $storeScope);
                $senderMessage = $this->scopeConfig->getValue(self::XML_PATH_SENDER_MESSAGE, $storeScope);
                $order = $observer->getEvent()->getOrder();
                $custFirstname = $order->getCustomerEmail();
                $custFirstname = $order->getCustomerFirstname();
                $custEmail = $order->getCustomerEmail();
                $template_sub = $templateSubject; # Set email subject
                $sender_name = $senderName; # Set sender name
                $sender_email = $senderEmail; # Set sender email
                $template_content = $senderMessage; # Set email content
                $templateId = 'cancel_order_template'; #This is my custom template ID
                $this->inlineTranslation->suspend();
                $storeScope = \Magento\Store\Model\ScopeInterface::SCOPE_STORE;
                $sendFrom = $sender_email;
                $senderName = $sender_name;
                $sendTo = $custEmail;
                $transport = $this->transportBuilder
                ->setTemplateIdentifier($templateId)
                ->setTemplateOptions(
                    [
                        'area' => 'frontend',
                        'store' => $this->storeManager->getStore()->getId(),
                    ]
                )
                ->setTemplateVars([
                    'template_subject' => $template_sub,
                    'email_content' => $template_content,
                    'order'=> $order
                ])
                ->setFrom(['email' => $sendFrom, 'name' => $senderName])
                ->addTo($sendTo) //I make change here and it's working now.
                ->getTransport();
                $transport->sendMessage();
                $this->inlineTranslation->resume();
        } catch (\Exception $e) {
            $this->logLoggerInterface->debug($e->getMessage());
        }
    }
}

Because there is one if condition where isset function is used and whenever we will use addTo() function with string type of argument at that time that is working because first condition become true there. But when we will add array as an argument in that function at that time when we check $this->messageData there is not any index with to addressType. So I just initialize that index at starting of this function and assign blank array as an argument. So now whenever we will use array as an argument second argument become true in that case and we can send multiple emails.

Hope this will help you!