From Tuesday 3 November 2020, Microsoft are withdrawing Office 365 Pro Plus support for all versions of MacOS older than 10.4 Mojave.
Anyone wanting to install or update Office 365 Pro Plus on a Mac will need to upgrade to 10.14 or later. If you have an older version of macOS and already have Office 365 Pro Plus installed, you will be able to continue using your Office apps but will no longer receive any updates including security updates. It is therefore important that you upgrade macOS so that you can continue to receive security updates for both Office 365 as well as your operating system.
Office 365 Pro Plus or Microsoft 365 Pro Plus includes Word, Excel, PowerPoint, Outlook and OneNote and can be downloaded from portal.office.com