Thanks T-Mar for that info. I suspected cleats went back that far because there are clipless pedal patents dating from the 1890's, but I did not know for sure.

If you've ever ridden a fixed gear down a mountain it becomes immediately obvious that foot retention would have been a major motivating factor in the early days of cycling.

AFA riding with slot cleats - it really was not a big deal. You got used to it after a ride or two, and it became instinctive to flip open the strap of your lead off foot. Functionally cleats and straps were basically the same as riding with modern cleat shoes. Generally straps were kept fairly loose in town, and then tightened up when you got on the main part of the ride and the paceline formed up or whatever.