Here in Germany, the Autobahn has a minimum speed cars need to be able to go (60 kph) although it’s not a minimum speed you necessarily need to drive.

However, we do have a dedicated minimum speed sign, but it’s very rare and usually only used in very specific places. The only one I know is a long and windy bridge on the side of a small mountain where, depending on your lane, you need to go at least 60, 80 or 90, I think.