Edit the Systemd networking files at /etc/systemd/network/*.network and set all the ethernet and wireless interface to Unmanaged=false but do not touch the localhost interface or "lo". This will prevent systemd from automatically bringing them online but you still have to turn them off manually. Now run sudo ip link set down for all the interfaces you set to unmanaged to disable them and they won't be brought back up on reboot.
Edit the Systemd networking files at /etc/systemd/network/*.network and set all the ethernet and wireless interface to Unmanaged=false but do not touch the localhost interface or "lo". This will prevent systemd from automatically bringing them online but you still have to turn them off manually. Now run sudo ip link set down for all the interfaces you set to unmanaged to disable them and they won't be brought back up on reboot.