At the end of the month we will be moving to full fibre broadband having been on fibre to cabinet with digital voice of a while. Given we have digital voice with handsets connected to our Smart Hub 2, will I still need my old BT Master Socket once the ONT is installed?
No
They don't remove the sockets normally as cabling still there
My advise, don't remove the master socket, or disconnect the wires or switch over the hub to FTTP until you know you the full fibre install is working and stable for 24 hours.
If not, and it falls over due to a fault or Openreach mess up, you will need to switch back to FTTC on the hub to get connected again.