As David has pointed out, British 13A sockets have had shutters since they were introduced in the late 1940s.
I'm not sure when shutters were first fitted to BS546 (round pin) sockets either. Could it have been earlier? It certainly isn't a bad idea with the 15A types, as the pins and socket apertures on these devices are really large, and could easily accept a metallic object the diameter of, say, a pencil.
For many years (decades) the shutter mechanism was operated by a pin which extends into the top (earth) contact so that as a plug is inserted the shutter is pushed downward to open the phase & neutral apertures (a few types operated in a slightly different way, but still operated from the earth pin). I'm sure David will confirm that in the U.K. we soon master the knack of opening the shutter with meter probes!
Some more modern types have a rotating shutter pivoted between the phase & neutral and need equal pressure on those two pins before it will open. Still possible to get in with probes, but a little trickier.
I believe that some manufacturers (MK?) now have 3-way shutters which won't open without equal pressure at all three points. I've not encountered one of these yet -- Looks like probing will get more difficult though!
