There is the ability to expand your inbox per session and not see the side panel, and you can also use various extensions to block the panel all the time for you.
Along comes the new banner which floats on top thel inbox. It is extremely annoying. The good news is there is an extension that will get rid of it and the side panel.
Yahoo Mail Hide Ad Panel , by Piyush Soni latest ver 2.43 was released July 23, 2014. I added it to my Firefox browser and it is working very well
For Chrome, there is Hide Inline Yahoo Mail ADS 1.0.3 by Alpha Charlie, released July 7,2014,.I gave it a try and it does work well, however there are few users and not yet rated.