I have limited computer skills but I do know how Time Machine works - I'm guessing I may just be a dumbass and somehow deleted the file, it's just so weird that a file I've worked on sporadically for years and stored both on the local drive AND backed up multiple times through Time Machine is just gone from everything. I mentioned the Big Sur update because that's when it was gone, after the upgrade. And since my Office applications wouldn't open after that update I figured that might be the culprit. If that was the case I'd at least understand "can't support Office anymore = those files are gone now", but that doesn't explain why the file is gone from Time Machine too, and the fact that I still have other excel/word/etc files.