Yesterday, CCC released version 5.1.23 which will create a bootable backup for BIg Sur on Intel Macs. The first backup under this release asks if you want the backup to be bootable. If you say yes, the backup will take a while as it needs to recreate the system volume.
Note that you still cannot create a bootable backup for the new Apple Silicon Macs, but the backup created is fine as a source for doing data restores. See the release notes for details.