Here is a little bit more from the other thread that is relevant here.

First, Microsoft is not reliable when they say something indicating that a function will soon be unsupported. An example is the GlobalAlloc function, in which (in the other thread) I quote the MSDN saying that it is provided only for support of 16-bit Windows and now it says something different. I also say that RegOpenKey is used by MFC in more than a dozen places. Therefore we probably should submit something to MSDN saying that the documentation needs to be revised.

I also say that therefore if RegOpenKey is not supported by any version of Windows, then that version will also not support programs compiled using versions of VC 6 at the earliest and probably the later versions of MFC too.

Another point made in the other thread is that the registry did not even exist for most of 16-bit Windows.