It's the result of any email being failing to send.
It is being generate by the control panel configured crons, both the Wordpress ones and bespoke ones.
I'm not on Fridays version yet, but I notice the "Request URL" cron method is still broken.
dead.letter will continue to fill up with every cron run that has output.
I'd be surprised if cron wasn't already something on the enhance roadmap and subject of support tickets, so I've not raised a ticket myself.