I think the better approach is simply showing the "Intended for johndoe@example.com" next to Unsubscribe, but I could see why they ask for your email.
List-Unsubscribe: <mailto:list@host.com?subject=unsubscribe>
I don't see why you have to involve the web at all, but I can tell you that if I have to go through a bunch of bullshit when I want to unsubscribe I'll just mark it as spam instead. However appreciated and anticipated your newsletter is, you have to consider that most newsletter subscriptions are probably either accidental (failed to uncheck some box when signing up for something entirely different) or straight up unsolicited, and people like me will basically purge all their subscriptions without discrimination regularly as the crap builds up.
Because List-Unsubscribe is very new and not supported by all email clients or ISPs (inbox service providers). Since adoption is so spotty, most ESPs (email service providers) start from a baseline of a web unsubscribe system and (maybe) supplement that with List-Unsubscribe as well.
This situation will likely improve over time.
Adoption of it doesn't matter insofar that you can use the same link in the body as a fallback, which was part of my suggestion.
"ESPs" don't care because cumbersome and convoluted multi-step unsubscribe is a selling point for the majority of people who use such systems. They have moved from simple mail based unsubscribe which was the norm for years.