During WW2 the vehicles I've seen mostly used by the US, namely Shermans and Jeeps, had proper old fashioned ignition keys (though I do know of a few Shermans that seemed not to follow this trend). While this is by no means a guarantee, I'd think WW2 would have had mostly keyed ignitions.

More recently, in Iraq, the humvees are now obviously almost all using a three point switch to turn on and off, as seen here. This offers notable advantages in starting the vehicle in combat scenarios, being much quicker. That post and photo were made in 1998, likely much earlier. So I'd think the Humvees used by the military used to have switch ignition since their entrance into military service in 1984. The only book I have on US military vehicles fails to mention this fact as a common thing for all US vehicles, but does note it for Humvees.

With this much info I can narrow down the timeline for switch ignitions for Humvees only to '1984'.

Hope this helps. It's an odd topic to ask a question about, with not as much ready info as I'd like to have.