My answer will be:
I' am yet to confirm who that Slave is. Remember the slave is appointed by Jesus to care for their assignment. This slave would be a class of divinely appointed Stewards IMO. They will be like the apostles. Just as Paul was appointed as an apostle to preach, form congregation and teach them, that's how i see the slave to be. In our day, this would be in form of modern General Overseers. As Paul was sent to form churches and teach them, these overseers are divinely appointed to form churches and teach them.
However, I don't know for sure who has received that assignment from churches available. I would welcome a miraculous evidence for such appointment.
The GB think they are that slave because they think that the way the first century christians functioned were through the direction of a GB in Jerusalem. They are wrong.
Jesus appointed the apostles who go and establish congregation all over the world. They cared for those group they formed, wrote to them, appointed overseers, answered their bible questions without seeking approval from any GB from Jerusalem. If you read 1cor very well you will understand Paul received a question from the Corinthians, and he answered it in that letter, giving them direction about worship, spiritual gifts, disfellowshiping, resurrection, marriage, singleness, etc He didn't request approval from Jerusalem to send his letter. He was a Steward appointed to feed those congregation he he was divinely appointed to form.
I feel any such Slave today would run similarly.
Secondly, bear in mind that attaching "Faithful and Discreet" to the word slave is tricky. Jesus would be the one who will declare them faithful and discreet when he comes. That phrase is not a title. Jesus used that phrase to show that they were faithful when they were appointed, but whether they will remain faithful is another thing. If the slave became sluggish or start to beat his fellow slave, he is no longer faithful and discreet. So it is wise to wait when Jesus comes to know if one was faithful and discreet.