Are you using a custom theme? If so check your shipment tracking email template in your child or parent theme :
There is an issue with deprecated code in older email templates.
Edit track.phtml and around line 28 replace
<?= $block->escapeHtml($_item->getNumber()) ?>
The preference mechanism doesn't work on abstract classes :)
When you have such a scenario, you should look for public methods that use the protected ones and alter the behavior there with a plugin. For example, in the class you mentioned, I found the method getProcessedTemplate() which uses the protected methods responsible with setting the logo - you could ...
First of all, you have to check 'who' sends the email template and see which are the parameters sent to the template, so that you can grasp the way you can call your attribute by either accessing the existing customer object or sending your custom object or parameter.
I suggest you check the class app/code/core/Mage/Customer/controllers/AccountController.php ...