Your question asks if the beautiful twenty-third Psalm is still relevant today. I can only speak for myself, but I find its comfort and reassurance very helpful as I move through the challenges of this human experience we're sharing.
Metaphysically we understand the Lord to be synonymous with the Christ—the indwelling Presence of God that is the truth of our spiritual identity. The Christ in me is infinitely powerful, but it's a quiet power; it doesn't shout to get my attention the way some of the negative thoughts in consciousness do. That's why it's important to remember that it is always within me, available to "shepherd" me lovingly through the shadows of death and evil that seem so real, to the green pastures and still waters of peace, joy and abundance that are my destination. The "goodness and mercy" of the Christ are with me all the days of my life; it's up to me to know that and to choose always from that rich resource.