From what I know, S2 can be implemented in the 500 series chips. There's not a hardware limitation there.
The 700 series just has better performance for range, speed, etc. Probably handles S2 better, and maybe that's why some companies are waiting to implement S2.
For Example: the Zooz ZEN26/27 switches implement S2 on 500 series silicon. I think Schlage's Zwave Plus locks do as well.