I remember Dead Letter Queue gets populated when some queue or topic fails internally when a job/task is running not that you can put your own stuff there.
No, we have not any plan to adding that. The connection for MS Azure Service Bus for Litium is not upgraded to Litium 8 and the recommendation is to use our Redis implementation instead.
When using the Microsoft Azure Service Bus the dead letter queue is used for messages that not is understandable, example malformed json or references to a type that not exists. If an exception is thrown during execution the Abandon method is invoked that stops the current execution of the message. When the retry count for a message is reached the Service Bus will automatic put it in dead letter. If the project not handle the dead letters manually they will be removed automatic from the dead letter queue regarding to the settings on the queue.
When using Redis the concept of dead letters does not exists and messages that is malformed will be sent to the error log. If an exception happens during execution the message is logged and if not the retry count for a message is reached the message is put back into the queue again, otherwise discarded.