Yahoo! now notifies users via email that their Yahoo! account was added into OS X.
This never happened before. This happened today just seconds after I finished upgrading my MacBook to OS X 10.11 El Capitan.
When the new OS X booted up and connected to internet and I was using Yahoo! account in Mail or Messages, Yahoo! automatically notified me via email that my Yahoo! account was added into OS X.
This would be a good feature for users to let them know that their account has been used by someone. If they do not use it, it means that the account may be hijacked. The email notification provides a link to reset password.