When it’s a Gmail period or plus alias, you don’t need to confirm. I just tried it and it only took 12 seconds.
Yes there are other kinds of weird delivery setups. The devs have already said they’ll eventually make it easier for admins (and maybe users) to add secondary emails for users. I don’t think they should go out of their way to do any more than that. Users should take responsibility for their own email setups if they want to reply to email received at an address they can’t send from.
As an option admins could enable, the token idea could work. We already trust that if users forward their forum emails to others then they could unsubscribe them using the links inside. Of course posting is potentially more destructive than unsubscribing.
As a feature request, the devs often say they have a rule of three, where once three people have request it they’ll start considering it more. Or if you’re a paying customer.