I'm thinking mailman3 should probably keep a list of user bounces. So that when a user finds out he got unsubscribed, and logs into the interface, mailman could shown him the evidence on why it got disabled: «see. the last dozen emails I sent you, at dates D₁... D₁₂ failed with reason "no such user", "mailbox is full", "i'm a teapot", "This looks like spam"...  What did you expect me to do if you don't accept my mails?» So that the user could confront their mail provider on why it is rejecting the emails he wants to receive.

Best regards

PS: Join me to the list of guys congratulating a good work on migrating the lists!