Yahoo disabled forwarding
There is, of course, all sorts of speculation as to why forwarding has been disabled including speculation this has to do with holding on to accounts during the Verizon purchase. It’s certainly possible this is the case.
However, forwarding email is hard. Forwarding email on a large scale can result in spam blocks and delivery problems. It’s such an issue M3AAWG published a forwarding best practices document. It’s possible that Yahoo is making some changes on the back end to better implement the best practice recommendations. I don’t know, but it’s possible that Yahoo is telling the truth that they’re improving technology.