Mike Bombich, writing on the Carbon Copy Cloner blog:
For decades, Mac users have taken for granted the Mac’s “External Boot” feature. Prior to Mac OS X, people could simply drag and drop the System folder from one volume to another; presto, external boot volume. When Apple made it more complicated with Mac OS X, we pioneered the “bootable backup” solution (nearly 20 years ago!), and this has been a feature we’ve reliably supported on every new Mac and every new OS since then.
But Apple has never been afraid of shaking things up to blaze new trails. Big Sur and the new Apple Silicon Macs have shaken up the way Mac users will recover from hardware failure.
As he writes, it’s becoming clear that bootable backups are not long for this new Apple silicon world. Apple told him that “copying macOS system files was not something that would be supportable in the future.”
Having a bootable backup is something I’ve done for years, and keeping one around can be a great troubleshooting tool. It’s a bummer to see it go away, but as Apple continues to make the Mac act more like its mobile siblings under the hood, this shouldn’t have come as surprise.