The term generally applies to the early front derailleurs, from Simplex and others, which were a single component comprising a lever, a pivot, and the cage itself. One had to reach down the seat tube to change gears; hence the "suicide" moniker.

Conventional downtube levers are a BIG improvement.

By the way, I also consider the old normal-low bandspring rear derailleurs "suicidal," because cable failure can throw the cage into the spokes.