MACUPDATE: Beginning w version 16.67, Office now requires macOS 11 (Big Sur) or later. Like many vendors, this is based on a policy of supporting the current macOS and two previous releases. Users needing version 16.66, the last version to support Catalina (10.15) can find it here
https://macadmins.software/
This is a legit site for IT professionals, all download links are direct to Microsoft servers.