The first Arduinos with that formfactor which supports shields are from around 2005[0]. The Uno wasn't the first[1], it was just a model line set up for that specific formfactor for consistency, since the name was changing frequently (e.g. "Arduino Diecimila", "Arduino Duemilanove", etc).

[0] https://docs.arduino.cc/retired/boards/arduino-serial [1] https://docs.arduino.cc/retired/other/arduino-older-boards