TL;DR: it's not the sin itself which is "unto death" or "not unto death", but the attitude of the sinner that makes it so.
We know that we are all guilty of sin:
For all have sinned, and come short of the glory of God — Romans 3:23
We know that sin leads to death:
For the wages of sin is death; but the gift of God is eternal life ...
Here is a quote from Dr. Bruce Milne, formerly lecturer in Biblical and Historical Theology at Spurgeon’s College, London. The Foreword to his book is by J.I. Packer. I believe this is still the current Reformed Protestant view:
Recent interpretation sees the sin as essentially Christological. Jesus distinguished between sin against the Spirit and sin ‘...