Microsoft decided not to type "make" for NTVDM on 64-bit versions of Windows (I would argue arbitrarily). It has been unofficially built for 64-bit versions of Windows as a proof-of-concept: https://github.com/leecher1337/ntvdmx64
That’s okay, and if people want to test their specific use case on that and use it then great.
It’s a pretty different amount of effort to Microsoft having to do a full 16 bit regression suite and make everything work and then support it for the fewer and fewer customers using it. And you can run a 32 bit windows in a VM pretty easily if you really want to.
Not sure exactly what it does (other than obviously being some variation on process creation), but the existence of a function whose name starts with NtVdm64 suggests to me that maybe Microsoft actually did have some plan to offer a 64-bit NTVDM, but only abandoned it after they’d already implemented this function.