Yahoo and AOL mail were acquired by Verizon back in 2016. While this was a welcomed change as Verizon would reinvigorate the product and would bring much needed security updates it was bound to mean other changes.

 

Effective January 1, 2021 Yahoo mail will not automatically forward emails to an alternate address unless you pay for the the "Pro" product.

 

While Yahoo is stating the change is being made for security reasons, which I doubt as they are offering it for a price, the more likely reason is the desire to move free clients to a subscription based, revenue generating product.

 

You can read about the changes here.

 

Automatic Forwarding of Emails