Email Sort by Sender is gone. This useful option should come back.
Please restore Sort by Sender in the email sort options.
23rd
ranked

Thank you for your feedback.
If you are using the Conversations mode, sort by sender is not available. If you want to use sort by sender, you will need to turn Conversations off in settings:
https://help.yahoo.com/kb/new-mail-for-desktop/manage-conversations-yahoo-mail-sln28055.html
Thank you!
Mail Support