After finally finding a link on Yahoo mail that would allow me to email customer service rather than just posting to community, (they are no longer taking calls at their customer care phone number), I got a response and miraculously my email was working correctly again at the same time. It really does seem from their email text (below), and the speed of the "fix" that they are possibly just trying to force people to use their new app.
Yahoo response: "We're sorry to have to hear from you like this, but thanks for letting us know you're having trouble getting email using the default mail app on your Apple device. We've received other reports just like yours, and our engineers are working hard to fix this as soon as possible. Keep in mind that you can still access your email through the Yahoo Mail app or mobile web site. Thank you for your patience as we work to get this resolved for you."