Security Updates are only provided to the current system and two previous, but there is evidence that Apple doesn’t always patch all vulnerabilities on those two previous.
One other consideration is that Big Sur is generally much more finished and stable today than Catalina will ever be. That’s not to say that there aren’t some hurdles with Big Sur, but most users found it to be a smoother update than many previous ones have been. You’ll find a recent discussion of Big Sur issues in [Small] Annoyances in Big Sur.