Archaic KJV Word
Tares
Modern equivalent: weeds
What Was Lost
The indistinguishability. Tares (darnel) look exactly like wheat until the grain heads form. Their roots intertwine so deeply that pulling tares would destroy the wheat. Jesus's point was devastating: good and evil in this world are so intertwined that premature separation would destroy both. Only the final harvest reveals which is which.
Closest Survivor in Modern English
tares (still used but only in quoting this specific parable)
Peak Usage (1611)
KJV Matthew 13:25 -- 'His enemy came and sowed tares among the wheat'
Died ~1900
Greek zizanion ('darnel/poisonous weed indistinguishable from wheat until harvest') fell from common usage as urbanization disconnected people from agriculture. The terrifying indistinguishability of good and evil was lost.
What Replaced It
“weeds”
Obviously different from crops; tares were identical to wheat until the grain appeared -- the whole point was they looked the same
“bad plants”
Generic; zizanion was specifically a wheat-mimic whose roots intertwined with wheat's roots
“imposters”
Detectable; tares were undetectable until the final harvest revealed the truth