For whatever it's worth, I got Yahoo! email to update on my iPhone X after:
- I turned off Yahoo Account Key
- I turned on the 'allow less secure whatever' setting on Yahoo account
- I quit the IOS mail app
- I deleted the account from IOS
- I manually rebooted the phone
- I re-added the Yahoo account (using the Yahoo account choice, not manual add)
- I turned Yahoo Account Key back on
I have not given it a day yet, as others have experienced the '24 hour and gone' phenomenon, but I'm able to get emails to my Yahoo account through mail app right now, fetching automatically with badge updates. It's worth noting that my old iPhone 6 is working fine for Yahoo mail through this whole ordeal, and also worth noting that just deleting the account and adding back (even with quits and reboots) did nothing for me, so I suspect that the Yahoo settings is ultimately what did it. The only thing I did not test was the last step, but so far, turning account key back on hasn't stopped emails from coming in. Good luck to you. I'm hoping that this fix sticks.