Starting with the Masterpiece line in 1946, all K frame guns with adjustable sights had the trigger stop-
It is adjustable. It is possible to move it into a position that will NOT let the trigger release the hammer. It is held in position by a screw under the sideplate:
I THINK it was in the 80s, but it MAY have been the late 70s that a story started circulating that a trigger stop came loose on a cop's gun and moved into a position that would NOT let the gun fire and the cop got killed in a gunfight. I do not know if that is truth or rumor, but I DO know that many PDs started removing the trigger stops from duty weapons.
S&W recommended doing so.
Not long after that, I do know for a fact that 38 and 357 K frames started shipping WITHOUT trigger stops. They still had the slot for the stop, but it was not included with the gun. Instead, you got this piece of paper:
I think the Mod 17 and 18 still shipped with the Trigger Stop. Probably the Mod 14s also.
Sorry, but my knowledge of more modern guns is pretty light, so I can't tell you when/IF they were dropped or current status.