They’re still there in South Dakota. They put up the dual unit signs for a few years starting about when the 55mph speed limit law ended.
I was in Germany back in 2000-2001 on business trips and had to drive on their roads with solely kph units. Of course the speedometers were also solely in kph units, also. I usually just kept up with the traffic in town and didn’t worry about the speed. On the Autobahn, outside the city limits, there is no speed limit so it didn’t matter anyway. I had to do mental math to find out how fast I was driving in mph using the old 88 kph as a reference for 55 mph. So when the speedo said 190 kph I knew I was doing about 110-115 mph.................