"Newer" is not a synonym of "better"!
I had email that was meaningful to me going back many years and now I can't find it. Who the told you that was a good idea? It certainly wasn't the users.
Yahoo has a long history of stupid ways to * up everything that they finally got right.
You have also taken away the display of how many messages were in the mailboxes, and the task buttons at the top of the mailboxes are almost invisible except in dark. Not only that, but the checkboxes for individual groups of messages are also gone.
I found a couple of good things that I expect you to f**k up soon as you can so i'm almost afraid to mention them. I like having the number of messages in a conversation listed alongside the title of it. The return to top arrow is also good.
Protonmail, here I come!