Laravel has a feature called "On Demand Notifications", that allows you to send a notification to a notifiable that is not backed by a user model.
Notification::route('mail', 'taylor@example.com');
->notify(new InvoicePaidNotification($invoice))
When sending an on demand notification, the resulting entry in the notification_log_items table, will have notifiable_id and notifiable_type set to null. You'll find the configuration of your on demand notification in the anonymous_notifiable_properties property.