Thanks for the research and links, @mark2.
I will note that at this CCC Knowledgebase article (updated 13 days ago), @bombich writes:
Copying Apple’s system is an Apple-proprietary endeavor; we can only offer “best effort” support for making an external bootable device on macOS. We present this functionality in support of making ad hoc bootable copies of the system that you will use immediately (e.g. when migrating to a different disk on an Intel Mac, or for testing purposes), but we do not support nor recommend making bootable copies of the system as part of a backup strategy.
Please bear in mind that you can restore all of your documents, applications, and system settings from a standard CCC backup without the extra effort involved in establishing and maintaining a bootable device.
I will admit to some real uncertainty here. I felt that this was important news to get out when I came across it shortly before the issue deadline, but I haven’t had a chance to contact either Mike Bombich or Dave Nanian to learn more.
Although I’ve been making bootable backups of my 2020 Intel-based 27-inch iMac with SuperDuper for a long time now, I haven’t checked to see if they will actually boot it. And my understanding is that it’s entirely different on M-series Macs.