I doubt this is true, but I can't confirm it. We get ROC/ROD from all MODE S readouts if we get anything at all. I doubt that they are all ADSB equipped, but perhaps they are.
If you are seeing ROC/ROD for aircraft not equipped with ADS-B, such as the types I listed, then it's being computed from successive DF4 (altitude interrogation) Mode S responses. In the absence of ADS-B, there is no provision for the aircraft to send explicit vertical rate information.