The #1 feature request is for Bitwarden to be able to auto-fill Windows (and Mac) desktop apps. “MS Office you download or buy off the shelf that runs on my PC” is an installed “desktop app” so, no, auto-fill does not work in Office 2021 on Windows 10.
Office 365 can operate either as an installed app (no auto-fill) and it can operate as a web app (yes/maybe auto-fill).
I was focusing my response on Windows 10, as indicated in your title, but you are right that Auto-fill (mostly) works on the mobile apps.