Archaic KJV Word
Witness
Modern equivalent: observer
What Was Lost
The life-cost. Greek martys became English 'martyr' because the earliest witnesses were killed for their testimony. To be a witness was not to passively observe but to actively testify with your life on the line. 'Ye shall be witnesses' was not 'you will watch' but 'you will testify with your whole life, including your death if necessary.'
Closest Survivor in Modern English
martyr (preserves the life-staking dimension but only in the extreme case)
Peak Usage (1611)
KJV Isaiah 43:10 -- 'Ye are my witnesses, saith the Lord'; Acts 1:8 -- 'Ye shall be witnesses unto me'
Died still used but passivized (~1900)
Hebrew ed and Greek martys ('one who testifies from personal experience/one willing to die for testimony') passivized into 'someone who saw something happen.' The active, life-risking dimension of testimony was lost.
What Replaced It
“observer”
Passive watching; a witness was an active testifier willing to stake their life on their testimony
“bystander”
Accidental presence; a witness was deliberately appointed and publicly accountable
“spectator”
Entertained observer; Greek martys became 'martyr' because witnesses died for their testimony