It is a little technical but search on MCP or Vertical and you will see what is in the ADS-B standard as implemented by EUROCONTROL.
Actually, it is a technical specification, but even that does not support selected mode (V/S, open descent, managed/VNAV and so on). And secondly ADS-B is not implemented in europe yet, not certified and not required. However, Mode-S Enhanced Surveillance is, which doesn't support vertical mode either. Both of course transmit altitude target, which is the value that FR24 decoded in the datastream as well.
And even though a 25 year old aircraft had to be retrofittet to conform with Mode S EHS, it is doubtful it would be retrofittet a couple years later to conform with a 2014 non backward compatible specification that is not even required yet.