Jonathan,
To me this seems like a poor way to handle this issue, but it is a
complex topic and I'm not sure what would be a good solution.
I agree with J about leaving this part out. It would be fine in discussion, but I'm not sure you want this message to start that discussion.
As you note, it is a thorny subject and discussed at length in beta@. The short of it is Mark feels that certain of the email services have left him no other option to avoid having messages from Groups.io more likely sent to spam, affecting all users of that service not just this one member.
Things may indeed improve in the future, if the "one-click" unsubscribe proposal is adopted by many email services, and implemented well by them. But until then the best advice is to never mark a Groups.io message as spam, even if it is - you'll get unsubscribed either way.
Also, as J noted, if your email service automatically sorts some messages to your Spam folder (as most do) never allow any Groups.io messages to linger there. Mark them "not spam" before filing or deleting them. This implies a need to check your spam folder more often than the automatic deletion interval.
Instead take any spam messages up with the admins of that group. They may already know, but the likely case is that they'll have to do something to prevent more spam being sent through their group.
Shal
Shal Farley