Hmm. Sorry you lost all that.

I'm looking at my own files, and I didn't see anything under Spelling or UProof that looked like it contained my custom words.

Mine seem to be under c:\Users\[me]\AppData\Roaming\Microsoft\Office\16.0\ - there are two randomly named subfolders there and in each is a file called RoamingCustom.txt. By looking at the words, it appears to be a shared dictionary between Word docs, possibly Excel and probably Outlook, since I see some surnames there who are probably only people I've Emailed and not written about in Word.