Office 2021 for Mac
While the initial build of Office 2021 for Mac will install and work on macOS 10.14 Mojave, in December of 2021 macOS 10.15 Catalina or later will be 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'll no longer receive any updates including security updates.
Upgrading your operating system to macOS 10.15 or later will allow Office updates to be delivered for your apps. Note that new installs of Office 2021 for Mac will also require macOS 10.15 or later.
Microsoft Office 365
While the current build of Microsoft 365 for Mac will install and work on macOS 10.14 Mojave, in December of 2021 macOS 10.15 Catalina or later will be 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'll no longer receive any updates including security updates.
Upgrading your operating system to macOS 10.15 or later will allow Office updates to be delivered for your apps. Note that new installs of Microsoft 365 for Mac will also require macOS 10.15 or later.