Howard Oakley, on eclecticlight.co:
[…] Big Sur is either 10.16.0 or 11.0.0, depending on where you look. If your conditional code has only been checking for minor version numbers, then it would see Catalina as 15, but Big Sur could be either 16 or 0 depending on how Apple numbers it. If the major version has indeed incremented for the first time in the last 20 years or so, and Big Sur turns out to be 11.0.0, then anything which assumes that this minor version number will be greater than 15 will break.
Something to consider if your hacks start actions weird on the new macOS.