Something Apple changes seems to be causing issues with the ability to get email.
I’ve been having this issue for the last two iOS updates so I find it hard to believe it’s a Yahoo! issue. Ironically, the post is from an affected user who is blaming Apple for the problem. The thread is marked as solved, with the solution post shockingly not even remotely mentioning a solution.
There are several threads discussing the problem on Apple’s official forum, with one of the oldest ones currently on its 9th page and almost 2000 users saying they have the same problem (the number on the ‘I have this question too’ button we are referring to here). The issue is being reported/discussed by Yahoo Mail-iOS users on almost every relevant forum online. But the problems still stands unresolved. For months now (we saw reports as old as from October 2017), Yahoo Mail users have been complaining sync issues with the service while accessing it from within the native Apple iOS Mail app.
Looks like Yahoo has taken a big decision, but hasn’t officially announced it yet.