> Idempotence is not the problem here, by the way. That just means calling the method twice has the same effect. But GET should have no side-effect, in an ideal world. Of course, in the case of unsubscribe links, it needs to have a side-effect to comply with the law.

Could you have the page redirect to itself with POST? Like javascript redirect or metatag. Browsers would do this, and there wouldn't be any different for users, but bots, slack and Safari probably wouldn't.