Meetups I was planning on attending have been cancelled because it was too much trouble for me and the other 3 people also planning on attending to carry out all the steps necessary to RSVP on meetup.com.

If I get a unique link and click it, it's me. You don't have to verify the login, because if I don't remember the password, I'm just going to reset it via the exact same email address I got the link from. Also, it's likely that if someone sends me a newsletter, I used a one off password for that site and I would have to do the reset anyways.

If the unsubscribe action chain is longer than setting up a new email filter - you're doing it wrong. Not the OP, but I do leave and add the address to auto-spam as soon as I need to log in / do something else.