I'd doubt this is anything approaching universal, but where I am there definitely were three digit signs in the early aughts, which now appear to have all been replaced with two digit ones. I don't know if this was for cost purposes or the proposed "high score" reason.

There was one right in front of the police station in the town I lived in back in the day, which was a full dot matrix display and didn't even have discrete digit slots. It could display other messages if it wasn't in speed readout mode. I passed it at about a buck twenty one night and discovered that it topped out at 99. It's the first one I recall seeing that didn't have a third digit, but then they became the norm pretty shortly thereafter.

The ones that are popping up like mushrooms around here now fit within the footprint of a normal speed limit sign and they have a dual color LED matrix that flashes a frowny face at you if you're 0.01 MPH over the specified speed...