> Imagine that you silently unsubscribed an open-source maintainer from all github notifications!)

Do open-source maintainers forward around their unsubscribe links in practice?

The other problem with email scanners clicking links automatically can be solved without prompting for the email address. One simple solution is: if the link is clicked within a minute or so after sending the email there's a chance the clicker is an automated system. Instead of unsubscribing right away, serve a HTTP POST form with a single "Confirm Unsubscribe" button. Normal users will rarely see the form, automated systems will hesitate to fire off HTTP POST requests.