This came to my attention again recently, and I actually had the bandwidth to delve into the details.
We’ve had a List-Unsubscribe header for ages, but there’s a strong push for having an actual one-click unsubscribe without a further confirmation step from the sender.
This is accomplished using a new header defined in RFC8058
List-Unsubscribe: <https://example.com/unsubscribe/opaquepart>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
Upon explicit user verification, activating this triggers a POST request from the mail client with content similar to:
POST /unsubscribe/opaquepart HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 26
List-Unsubscribe=One-Click
We might need to add support for this.