Apple did issue guidance (that CCC and Shirtpocket did not follow) about the pending change. At the Bombich blog, he wrote this:
Last year at Apple's Developer Conference, Apple suggested that backup software should use Apple's "Apple Software Restore" (ASR) for cloning APFS volume groups. Initially I dismissed this –
Then later he says this:
If that's the case – if this is not actually a bug and is actually an intentional change by Apple, then I would argue that this is far worse than a bug. First, if third-parties should not set or remove the SF_FIRMLINK flag, then that should be documented alongside the flag's definition (i.e. in /usr/include/sys/stat.h). Second, if you're not going to allow the setting of the SF_FIRMLINK flag, then the system call should return -1 and set errno to EPERM – reporting success and failing is reprehensible. Last, and most important – making such a change in a production OS release with no warning is openly hostile to third-party developers who were relying on the documented functionality.
(Emphasis his)
But Apple DID give a warning about how to make backups that he dismissed, so it wasn't with "no warning" at all. And while is true that documentation should be updated more concurrent to software changes, documentation very often lags development and release.
Basically the two developers ignored the suggestion from Apple, continued doing it their own way, now find that Apple has, in fact, made a change that renders their approach a fail, then blame Apple for not documenting the change in the online help files. And Bombich then says that Apple did this with "no warning" to developers, despite having already said that Apple suggested an approach that he dismissed a year ago. So is it Apple's fault that he ignored the suggestions? (And before you pounce, yes, I was a developer and a manager of developers for 35 years, so I know about errors, flags, documentation.)
The failure to report an error is the bug and needs to be addressed, or the documentation updated to reflect the change, but it's a bit of a stretch to try to dump all of this on Apple. One has to ask if Bombich ever tests its own software. If they had done a full regression test, including testing the creation of a bootable backup from scratch by making and then booting the backup drive, this problem would have shown up much earlier. Sorry, but this one is definitely on Bombich and Shirtpocket for failure to adequately test their products and dismissing the guidance a year ago.