Your Microsoft Office documents are fine, are still present, and will still work.
You had Microsoft Office 2011 or earlier, and you need the now-retired Microsoft Office 2016, the current Office 2019, the Office 365 subscription, or the upcoming Office 2021 package as that becomes available.
Or you can migrate to LibreOffice, which is free.
to iWork Pages, Numbers, and Keynote, which can read and write most Office documents. The iWork apps are free.
Office 2011 and earlier are 32-bit apps, and 32-bit apps are not supported on macOS Catalina 10.15, on Big Sur 11, and future macOS versions. macOS started its transition from 32-bit apps and systems to 64-bit apps and systems back around OS X 10.5 Leopard, and developers have been increasingly encouraged to upgrade their apps to 64-bit and to newer interfaces for the past decade or so, and users encouraged starting since 10.13.4 or so.
For end-users. these apps will have been showing deprecated warnings on macOS releases. Warnings indicating that the app won't work on then-upcoming macOS releases. The Go64 tool was and is also widely suggested as a means to identify the older 32-bit apps prior to upgrading to macOS Catalina or macOS Big Sur or (as available) later macOS releases.
Your payment a decade or so ago still works fine on the older macOS releases, so by all means continue to use those if you'd prefer. As for paying for software apps once and those apps then working forever for free and that across multiple platform upgrades and across the platform transition to Apple Silicon underway, well, that's not how the app world works, and never has been. More than a few apps are now moving to subscription models to make this necessity rather more clear, including Microsoft and the Office 365 subscription.
A Microsoft rep reported that migrating from the older 32-bit interfaces to the newer 64-bit interfaces was a substantial effort for them within their apps, too. Different apps and different developers have had different experiences with the 64-bit migration here.
But apps aren't free forever, across arbitrary upgrades, any more than any of us will work for free forever, after receiving our first paycheck.