Hello,
Starting with macOS Catalina, JackRouter (the coreaudio “bridge” that makes Jack visible to non-jack-native applications) does not work, i.e. only applications that use Jack directly can use Jack. This is not an SC issue.
It is possible to build SC with Jack support built-in on macOS (it then needs Jack to run and will not work with the soundcard directly), so that’s an option if a custom setup if desired.