You could always install the apk file of a previous version by downloading it from an apk site. Keep in mind that there is no guarantee that these apks files are malware-free, so do this at your own risk! Make sure the "Unknown Sources" box is checked under System Settings/Security. You'll probably need to uninstall your current Yahoo Mail app first before you install the older version.
Here's a link for version 2.6.6: Yahoo! Mail 2.6.6 | APKDAD