This might come true some day, but it seems not in C++20 and in recent static reflection proposals there was no way to distinguish reflected from non-reflected members in the same class.
I am hardly a C++ user nowadays, but such stuff interests me, because the Windows Development team managed to push C++/WinRT as replacement for C++/CX, but are declining any improvement to Visual Studio support (at least comparable to C++/CX) until ISO C++ gets similar capabilities to C++/CX.
So given that some C++ usage is required depending on which APIs you want to access from .NET, you can imagine there are many WinDevs not very happy with the downgrade in tooling support.
Back to your point, in what cases might such distinction be relevant?
Here, only one data member is meant to be serialized. The others members are there to accelerate lookups into the first data member. (Full disclosure: that structure isn't actually serialized yet, but the Arc80 Engine which uses Plywood has similar examples.)