A lot of people apparently didn't get notified. The platform regularly lost emails. (Fifteen years of being on it, such problems were not irregular.) Also, they did offer an export tool, but the results were/are incomplete.

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.