As a note, from my quick notes, the Mode 2 Cartridge boot is present on at least most all EU and US revisions of the MCD BIOS for official MCD units that I can think of. Can't think of a specific EU/US BIOS official BIOS that doesn't have it, but leaving a bit of room for error. It's missing entirely from all JP revisions. No guarantees that Wondermega or LaserActive support it, either, but I don't recall checking those. There's also differences even within region revision for the security code, so it's not all that usable for production code. A few years back, though, I did have someone run tests for me and confirm that the mode does indeed work on hardware, so it's definitely functional, if seemingly vestigial. I'd agree that it's likely an engineering debugging feature, or something in that vein.

I'll see if I can carve out some time to scrape through exactly which ones do and don't have it, as it's not hard to find once you know what you're looking for code-wise, and it's easy enough to spot when it's not there.
I discovered some possible mistakes when going over the Software and Hardware manuals of the Sega CD.

The Software manual seems to indicate that the Sub-CPU shouldn't be able to write to the Comm or Status registers...
But the Hardware manual seems to say otherwise!
I don't know if what was written in the Software manual was a mistake or not, but it is interesting nonetheless.

Another possible error I found was the exclusion of the GRON bit (bit $F) on the Stamp Data Address
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Yeah... the "software" manual has more errors than the "hardware" one. The communication status array bit in particular is amusing to me, because if the Sub CPU were not able to write to them, then it would totally defeat the point of having the kind of communication they wanted to have in the first place.