Dave,

I understand all this, but why do Yahoogroups, Google Groups and all
my other various email list providers continue to work well without
having this automatic unsubscribe?

Granted it is still speculation, but my belief with respect to Yahoo Groups is that they stopped upgrades before Yahoo Mail implemented the mechanism. Another possibility is that larger, longer-established services may get a pass from the email services. Or at least better information about the email service's intent when sending the reports.

Some other GMF members and I been trying to consolidate what information is available here:
https://groups.io/g/GroupManagersForum/wiki/Removed-for-spam

Eventually I'll want to add citations to it other than just the Wikipedia article, including citing the messages from Mark about it.

Shal