Archaic KJV Word
Gospel
Modern equivalent: good news
What Was Lost
The herald's proclamation. In the Roman world, euangelion was the announcement that a new emperor had taken the throne or that a great military victory had been won. Mark's opening line was a political and theological earthquake: this is the victory announcement -- God's King has arrived.
Closest Survivor in Modern English
gospel (still used but flattened -- 'the gospel truth' now just means 'really true')
Peak Usage (1611)
KJV Mark 1:1 -- 'The beginning of the gospel of Jesus Christ, the Son of God'
Died still used but diluted (~1950)
Overuse and genericization. From Old English god-spell ('God's story' or 'good story'), the word became a generic term for 'good news' and then a music genre, draining its specific theological content.
What Replaced It
“good news”
Any positive information; gospel specifically meant the earth-shattering announcement that God had acted decisively in history through Christ
“message”
Neutral and generic; gospel was a proclamation with the authority of a royal herald announcing a king's victory