I have two bootable drives using CCC. With Big Sur Apple made a change to what can be written to the system volume of the drive. Basically once the system is installed, nothing else can modify that volume. That’s why there are now two volumes, one with just the name of the drive, and the other with “ - Data” appended to it which is where your apps and files live.
CCC is able to create a FRESH clone of a drive, including the system volume. But it can no longer modify that volume on the backup drive once it’s created. It can only modify the “ - Data” volume. What does this mean in terms of practicality? You can absolutely create and “maintain” a bootable backup. What you cannot expect, is that any future OS UPDATES be applied to the backup drive (because they’re part of the OS volume, not the Data volume). But any changes made to the apps, documents etc. would be backed up since they reside on the Data volume.
I just had to boot from a backup drive yesterday, and it was exactly the same except running an older version of Big Sur. I can always boot from my backup drives and install the OS updates to them if I want to, but for a backup drive it’s not a big deal to me.