https://support.office.com/article/upgrade-your-macos-to-access-new-office-365-for-mac-features-f6efc6b6-90f8-4e69-82dd-d0de32a111aa

There is some indication that Office for Mac 2019 will only support the latest current release of macOS plus the previous operating system. This is a formal request from an Enterprise admin to have Microsoft provide support for the current macOS operating system plus the two previous operating systems.

Apple has been absolutely horrendous in documenting and providing the appropriate management tools to deal with major restrictions they've been adding to macOS. This makes it much more difficult to deploy the latest macOS release on day 1. As an example, the majority of our users are still on macOS 10.12 and we are just getting to deploy macOS 10.13 (almost a year later). And that's not for lack of trying or testing.