Microsoft will only support the last three current macOS operating system versions for Mac Office 2019.
The last three are the just released one and the two previous ones. According to this statement, Microsoft will start with this policy when Apple releases macOS 10.14 Mojave. Supported macOS versions will then be 10.14, 10.13 and 10.12. With the next big Apple update to 10.15, updates for 10.12 will be omitted and so on.
Microsoft says that the programs still run on older versions, but there are no more security updates. This applies to Office 365 for Mac and Office 2019. The 2016 still runs on older macOS versions.
