Carbon Copy Cloner (beta) can now make bootable backups of a Big Sur startup disk (on Intel-based Macs; see below about the new M1 Macs). In macOS 11.0.1, Apple addressed some of the problems with its APFS replication utility that rendered clones of the System volume non-bootable. If you have an existing backup task that's configured for a Data Volume backup, simply click on the Source selector and choose "Choose a different source," then select your Big Sur startup disk as the source to the task.
Bootable clones on M1 Macs are disabled for now because Apple's APFS replication utility does not currently support the special format of the startup disk's Data volume. CCC will automatically proceed with a Data Volume backup when backing up an APFS Volume Group on Big Sur. When Apple fixes this, we'll post an update to CCC that restores support for making bootable backups on these new Macs.