Using Mojave in lieu of Catalina

Mojave will receive another two years of security and browser updates before Apple support for it ends. At that time, with the advent of a new Fall introduced operating system, Microsoft will prevent installations of Office365 and Office 2019 for Mac on Mojave — because they follow Apple's support window.

Mojave can run on your current hardware as long as that hardware lasts and your bravery for no security or browser updates endures. Only Apple knows how long they will provide full installer downloads for Mojave.