Received this from Benny on how to auto delete.
It’s not a built-in feature. The best you can do is to setup a smart mailbox based on the Junk mailbox and let if have a Date-based condition, e.g., showing emails older than X days. Then add a rule to this mailbox (without any conditions) which is set to permanently delete the email. Make sure you set up this correctly since auto-deletion-errors can (obviously) lead to permanent loss of emails.
This smart mailbox will always appear empty since anything matching will be immediately deleted.
You cannot just add a date-based rule to the Junk-mailbox with the delete-action because rules are only triggered when something is “added” to a mailbox.