I'll need to check, but I think we want to add a full configuration section on Postorius for all the List-* fields (RFCs 2369, 2919, and now 8058). Besides the options to include the fields and set their content, there should be options for "Mailman standard" mailto and http(s) URIs, as well as an option to edit the http(s) URI for folks who use something other than Postorius.
Ugh, I had not considered that RFC 8058 breaks the separation of concerns here. The mail world keeps getting messily dragged into other infrastructure (MTA-STS, anyone?) -- we can at least make this work for Postorius users. Are there many who use just Mailman-Core and not the full suite?
I need to go back and read RFC 8058, but I think that the address information in the RFC 8058 URI can be completely opaque. I'm not sure if that's the best approach.
The URI path/query is defined as opaque. It probably should be something like list-name/subscribed-address plus a signed hash to avoid unsub-attacks. That means this likely will be a new web application endpoint (although it may be possible to overload the existing "unsubscribe" endpoint -- I haven't looked at the code yet).
--Jered