In an effort to make the answer more easily found for others who are also asking the wrong questions, and looking in the wrong places. And also for documentation purposes.
-M document-css=false
will stop pandoc from inserting CSS into the HTML header.
The long story..
An ansible script sneakily did a brew upgrade on a macOS dev machine just prior to a big sur update. It upgraded pandoc (which had not been upgraded in a while), then the OS got upgraded. After reboot, pandoc started inserting CSS into the header that broke the styling. I tried to run pandoc on a fresh install of linux mint with the latest updates and the latest version of pandoc to troubleshoot. It didn't insert CSS into the header, and the output styling was not affected. This further led me to believe the OS update broke something. I Grabbed a new mac that also had the new version of Big Sur, installed the latest version of pandoc, and it generated lots of CSS that broke the styling.
So yes John, you're 100% correct that it had nothing to do with the Big Sur update, and pandoc got upgraded. -M document-css=false proved that. Thank you, I owe you a drink!
-James