Yahoo retired its old Messenger protocol in favor of a new one, breaking compatibility with third-party applications, such as Pidgin, Empathy, and so on.
Eion Robb, the SkypeWeb and Hangouts developer, has created a replacement Yahoo prpl plugin, called FunYahoo++, that works with the new Yahoo Messenger protocol.
Note that I tested the plugin with Pidgin, but it should work with other instant messaging applications that support libpurple, like BitlBee or Empathy.
According to the plugin GitHub page, the new Yahoo Messenger protocol lacks quite a few features that were available with the old one, such as typing notifications, away / idle statuses, and bold / italic / underline formatting. Also, if you're previously used Yahoo Messenger, your old buddy list is no longer available. Since these are missing in the protocol itself, they cannot be added to FunYahoo++.
Furthermore, FunYahoo++ is pretty new and still needs work. For now it only supports basic features like sending/receiving messages and adding buddies. Two Factor Authentication is not yet supported.
Also, since both the plugin and the protocol are new, you'll encounter bugs. But if you want to use Yahoo Messenger in a desktop application on Linux, this seems to be the only way for now. The alternative is to use the web version.
If you cannot log in using the new FunYahoo++ plugin, you should try using the web version of Yahoo Messenger once, as that seems to initialize your account. Logging in using FunYahoo++ should then work.
You can report any bug you may find @ GitHub.
Install FunYahoo++ in Ubuntu or Linux Mint
To make it easier to install I uploaded FunYahoo++ to the main WebUpd8 PPA. To add the PPA in Ubuntu, Linux Mint and derivatives, and install the plugin, use the following commands:
sudo add-apt-repository ppa:nilarimogard/webupd8 sudo apt update sudo apt install purple-funyahoo-plusplus
If you don't want to add the PPA, you can download the deb from HERE.
For how to build the plugin from source or download Windows binaries, see the FunYahoo++ GitHub page.
After installing the plugin, select "Yahoo (2016)" from the "Add Account" protocol drop-down, as shown in the screenshot at the top of the article.