Yahoo finally got back to me with something other than a link to an out of date support article, saying they reproduced the problem and engineering is working on it and to give them 3 to 7 days.
That's disturbing either way I look at it. If the support person is telling the truth, that means s/he added my Yahoo account the his/her iPhone. If the support person is lying, well then s/he is lying.
Any way the response came back on December 26th and it's now the 31st. That's 5 days. I'd give them extra days because of the holidays, the problem started on December 19th. That's 12 days ago. I subscribe to the Yahoo Mail premium and that is really unacceptable.
For what it's worth the last time I had a problem sent to engineering it took several months to fix and that was a really bad problem (mail moved to folders on my iPhone would disappear). They did restore all the lost emails, but it took a long time.
As I mentioned before, this is only affecting my one account. A dummy account I created still pushes. Also Yahoo's iOS app pushes (for the most part). For now I'm using a combination of Yahoo's app and forwarding email to an iCloud email address. For what it's worth the Gmail app is pretty good (much better than Yahoo's app).