This was on the front page here in October when it was announced: https://news.ycombinator.com/item?id=21269614
Yahoo is definitely handling this badly: they should have offered an export tool, given a longer window, and been clearer in their messaging. But the claim that they didn't notify people (and just put the information up in a place no one would see) is wrong.
I'll add that some group owners/moderators have not even been able to log into their groups. Yahoo was doing something with needing a yahoo email to get in as admin, even when a non-yahoo email was associated with the account. Those people that lost access to their yahoo email account have been totally SOL.