With the lastest version of macOS Big Sur, 11.0.1, they just made this possible. To create a bootable external hard drive / clone with Big Sur, I used the most recent version of CCC, 5.1.23 beta, and was able to clone and boot from the external drive successfully.

You can now clone Big Sur to a bootable external hard drive. This happened with the most recent version of Big Sur, 11.0.1. I used CCC 5.1.23 beta (the current version at time of writing this), and was able to create a bootable external drive.

I just did this tonight, so will post instructions in case anyone searches this in the future. I used an early 2014 macbook air, and wanted to clone to upgrade from factory 128 gb drive to 1 tb samsung 970 evo. To make a bootable clone of my drive: My current version of MacOS was Big Sur version 11.0.1 (which this is the first version you are able to do this with). And I downloaded CCC version 5.1.23 beta. Formatted my external hard drive as APFS. Opened CCC, and followed directions on their webpage on how to clone my internal hard to the external drive. Drive should now be bootable. To install my new Samsung driveI switched out the internal factory hard drive for the samsung drive (note, my macbook air did not recognize the samsung drive at first, and I had to format it by putting it in a newer macbook, after this, my macbook air was able to recognize it.). Started my computer and held down 'option' key, and booted from the external hard drive. The OS was fully functional from this bootable external drive. I then opened CCC (which had been cloned onto the external drive), and cloned my external hard drive to the new internal samsung drive.