Yahoo announced today in a help forum that it is shutting down Yahoo Messenger in Yahoo Mail Classic. The company writes:
At Yahoo!, we’re focused on making your daily habits more inspiring and entertaining. That means we’re constantly reviewing our products and experiences and in some cases, have to make tough decisions to no longer support a product.
After much thought, we have decided to shut down Messenger in Yahoo! Mail Classic by March 22, 2013. This means that you will no longer be able to send IMs to your contacts from Yahoo! Mail Classic.
Yahoo suggests that you upgrade to the new Yahoo Mail, where Mesenger continues to be available or use the Yahoo Messenger client.