Yeah I'm stuck in Yahoo purgatory too. I have a Flickr account pointing at a valid email address, and an existing desktop and mobile session for that Flickr account. However, I can't actually create a new session by logging in because Yahoo wants to do its 2FA thing with a defunct email address. I can't add a valid email address to the Yahoo account because I need to create a new session to do this. The defunct address itself is no longer associated with the Flickr account and Yahoo/Verizon support has formally abandoned account support.
vOv