As of the November 2019 update for Office 365 for Mac or Office 2019 for Mac (build 16.31), macOS 10.13.6 or later is required to receive updates to Word, Excel, PowerPoint, Outlook and OneNote. If you continue with an older version of macOS, your Office apps will still work but you will no longer receive any updates including security updates.
Upgrading your operating system to macOS 10.13.6 or later will allow Office updates to be delivered for your apps. Note that new installs of Office 365 for Mac or Office 2019 for Mac will also require macOS 10.13.6 or later.
To provide you with the best experience, Office for Mac supports Apple’s three most recent versions of macOS. With the release of macOS 10.15 Catalina, Office 365 for Mac and Office 2019 for Mac supports macOS 10.15, 10.14, and 10.13. To continue receiving the most up to date Office 365 features on Mac, make sure you have one of the three most recent versions of macOS.
To learn more about the latest macOS releases, see this page.