[kirb]

That’s indeed what it is. It sends an automatic email to the List-Unsubscribe address, which if implemented correctly per the spec/regulations, authorises an immediate unsubscribe. More secure too because your email address is confirmed by SPF/DKIM. Nobody else with a copy of the email can unsubscribe you via List-Unsubscribe, like how just anyone can click the unsubscribe footer link if you forward it.