That's what I thought too, but it looks like it's just getting restricted to private groups and disallowing media/content uploads.

It's probably to push people off so they can shut it down later.

All messages will stop on Yahoo groups (last one of the bullet points.)

i would assume it's an anti-spam effort

It will just function as an alias for all that group members. Yahoo won't save the email on their servers; instead, expands the [email protected] to every member, and sends them a copy. That way, they don't need to maintain any infrastructure besides email server.