Where and how do I set the default setting of "Delivery status" to be "Enabled".
It already is. See https://gitlab.com/mailman/mailman/-/blob/master/src/mailman/core/constants....
At the moment, a new subscriber is neither "Enabled" or "Disabled".
That's because the member's list based preferences for that list are not set, so we fall back to address based preferences if set and finally to global preferences where Delivery status is enabled.
Log in to Postorius and select Mailman settings from the dropdown on the upper right at your user name to see more.
New list members don't have and list based preferences for that list and just fall back to their address based preferences or global preferences.
Ahhhh. Thanks very much Mark. I did some experimenting to get my head around what you've explained and I can see how it works now.