But even though they didn't really provide an explanation, I can see why they did make the change in question. This chart helps to explain. It shows my very rough estimate of the ratio of spam to legitimate discussion mail that's been affected. Literally many billions of email messages float out there throughout the internet, using fake Yahoo.com from addresses (and fake Hotmail/Outlook, AOL, Gmail, etc. from addresses as well). Being able to simply update a DNS record, and suddenly at least half of the top mailbox providers immediately start rejecting a big chunk of those malicious and unwanted messages.
In that context, the affected legitimate mail amounts to no more than a rounding error.
So while it's very frustrating for mailing list operators to have to change how their software works, let's not forget that this quickly and very successfully interferes with a broadly used spam and malware attack vector. For somebody like me, who cares a lot about stopping spam, I think that's a really good thing.