If it is a myth, it is propagated by the USPS. They will deliver for every mail address. USPS designated non-deliverable addresses are typically new addresses

If it is a myth, it is propagated by the USPS. They will deliver for every mail address. USPS designated non-deliverable addresses are typically new addresses that haven't been added, or as another commenter posted, an address that is unsafe to deliver. Now, delivering to your mailbox might be a central mail hub that is part of your community/neighborhood, and not directly to your door, but that is still your address's mailbox.