It would be sufficient to GET a resource that uses a script to POST the side-effect. Slack's user-agent probably isn't sophisticated enough to mess this up (although heaven help us when they implement their preview with something like headless chrome).
What would the problem be in just showing a button that says "Confirm unsubscribe" that sends a POST request? A lot of sites does something like that for their newsletter unsubscription.
I'm actually much more concerned when I don't have to click the link. It let's me know they haven't thought the problem through and what other errors and problems are there in their systems?
I believe asking to login violates the CAN-SPAM Act. That being said, there might be two links, one for "update commication preference" on top and "unsubscribe" in small text at the bottom. I always ignore the first and seek the latter.
