I don`t know why this idea with excluding the yahoo option when adding another existing mailbox to the mailbox list came to you. :(
(This option is only for the computer use. Not for yahoo on your phone.)
Now I cannot add my yahoo-mailboxes back to my list, after deleting them from my former list. This option is now gone, which is stupid. It's only possible for gmail, AOL or ather mailboxes to be added there, but not for the genuine yahoo ones. Very weird!!!
... because now I have all my g-mail-mailboxes in the yahoo app, but cannot get the yahoo mailboxes here. Clever!

More than that, the synchronization with the phone (where the yahoo app sends you to add these mailboxes) is not working. It has only synchronized one of all my 6 addresses and stopped. Now I was left out of all my yahoo mailboxes on my laptop. I tried a lot of ways to fix it but no one works.
Such a pity that after more than 25 years of using all my mailboxes in the yahoo app, I have to change it with another one.
:(