"Prepar3D is not to be used, offered, sold or distributed through markets or channels for use as a personal/consumer entertainment product."
Except for in the real world, where that's exactly how it is being used.
As far as I can tell most fsx add ons are now designed for fsx original, fsx steam and Prepar3d.
Personally I wouldn't bother with fs9 these days. A reasonably modern pc will run fsx fine. It's such a big franchise that it's the genre standard, despite being abandoned by Microsoft. It's essentially a flexible framework allowing third party developers to modify fsx far beyond it's origins.
For my money, A2A Simultaneous make the best addon aircraft.