Office 2011 will become insecure as Microsoft no longer provides important security updates for the suite. In fact, it’ll probably continue working for a while into the future. Microsoft Office 2011 isn’t going to stop working overnight.
So, if you’re still running Office 2011 you probably should be asking yourself the question, “What should I do?”. That means no updates or bug fixes, and if anything goes wrong or stops working in Word, Excel, Powerpoint, or Outlook, you’re on your own. Microsoft is no longer offering any support for Office for Mac 2011.