Spelling autocorrect and checking seem to have changed in Big Sur.
I have disabled Edit > Spelling and Grammar > Check spelling while typing, I unchecked System Preferences > Keyboard > Text > Correct spelling automatically, and yet I still get text underlined, for example, in Messages:
Am I missing another setting somewhere or is this a bug? In previous versions I could disable this easily.
Background: I use multiple languages and although in theory macOS can tell the difference, spell-check/autocorrect is not working too well with weird languages like Hungarian.
Best Answer
It seems to be limited to the Messages app (so likely a bug) and after a few times of quitting and restarting (interestingly one restart was not enough) it too seems to finally have learned the lesson.
So my recommendation to others with similar issue:
System Prefs / Keyboard / Text / Correct spelling automatically- this is either new in Big Sur (not sure) or it was toggled on again after update