As of the Office 365 for Mac September 2018 update, macOS 10.12 or later is required to update to the new version of the Office client apps for Mac and receive new feature updates. Users who do not update to macOS 10.12 or later before the September update will still receive mainstream support and will be able to continue using their current version of Office 365 for Mac.
As part of the upcoming September 2018 update, Office 365 for Mac users on macOS 10.12 or later will receive an update from the Office 2016 for Mac client to Office 2019 for Mac in order to maintain access to new feature releases and updates. Office 365 customers remaining on older macOS versions with Office 2016 for Mac will receive mainstream support, but not new Office 365 features.
Learn more about the latest macOS release at https://www.apple.com/macos.