Neowin published a story that Microsoft will drop support for Office 365 apps on macOS 10.13 in November.

Microsoft to drop support for Office 365 apps on macOS 10.13 starting in November

Microsoft is notifying Mac users that the Microsoft 365 suite of productivity apps and Office 2019 for Mac will stop being supported on macOS 10.13 High Sierra and older from November 2020.

Microsoft to drop support for Office 365 apps on macOS 10.13 starting in November