I have found a suggestion online that Mystic, Prologue and Specter were written by a third-party developer. Whether that is or is not correct, it may well be that the issue is that the code for these old (but clearly, to some, still valuable instruments) cannot readily be made Apple Silicon native. If so, I can understand why they were dropped rather than some factory content being Windows and OS X Intel only.
They are VST 3 in Cubase/Nuendo 11, so this is not because they are VST 2.