I don't like the new format. The following are the major issues.
Here is what I hate about the new format.
1. I always change the viewing options to view by unread messages. With the new format, I have to do that when going from 1 folder to another and then back to the inbox. I want an option to save that format.
I really hate the fact that the folders are not bolded when there are unread messages in them. I have a ton of folders and it is harder to notice the number next to the name when the name isn't bolded. Also it is missing the total number of unread messages next to the word Folders. This means it is easy to miss messages.
I kept refusing to "upgrade" but now it won't let me send messages. Anytime I try to send a message it says it may be spam but it isn't. That just started yesterday.

The key mission of Yahoo Mail is to deliver messages that people want to receive and filter out the messages they don’t. The easiest way to make sure your messages are delivered is to avoid looking like a spammer.
Please follow the steps here: https://help.yahoo.com/kb/review-practices-senders-sln3435.html