There has been much discussion which I won't repeat. Many commented that, for certain users, getting them to click a link in an email is a challenge (and I understand that).
I have 2 suggestions:
Suggestion 2 does not require the member to go to any web page, they stay in their email. They only need to click Reply and Send. I would suggest that the resubscribe command email does not generate the welcome message or an email containing a link to confirm - if the sender was previously subscribed to the group. This would require groups.io to retain knowledge that the member was unsubscribed from the group, essentially in suspended state as has also been suggested.