You haven't provided a plausible solution to this claim yet. So, for me, it's just an unproven assertion! Okay?
Furthermore, you haven't provided a single working solution for our situation. If anything, we've only received brief statements, no explanation of why or how!
The fact is, it didn't work here! Therefore, "maildir++ style .foo.bar folders are totally still accepted by Dovecot 2.4" is an unproven and boastful claim. I've now found a solution.
Your complete upgrade from 2.3 to 2.4, which was supposed to run better than major release 3.0, is a disgrace in terms of implementation and, above all, documentation. Given these facts, I would suggest you be a bit more modest and at least say thank you for the external support.
These kinds of half-baked claims really make me angry!
ByteMe