After the Office for Mac 2011 productivity suite support expires, users who do not have security and feature updates will have to choose a new version, such as MacOS 10.13 High Sierra’s Office for Mac 2016. However, it should be noted that 15.34 and earlier versions of Office for Mac 2016, and can not run perfectly in macOS 10.13. So even after the upgrade, they may not be able to use. In fact, Office for Mac 2011 should be in January 2016 to terminate the support, but because Microsoft was not ready for the latest Office for Mac 2016, so the former support time extended for 21 months.
Reference: Softpedia