The oldest I can think of is Namco's arcade game Bravoman / Berabow-Man, released in '88. A soft press would result in a regular punch, a hard press would have you stretch your arms to hit enemies further away. Supposedly the buttons would break a lot and have to be replaced often. Does anyone know of any earlier examples?
dsparil That's interesting. Looks like two big buttons with three strengths each. Must have made the game even harder to control, though the idea wasn't bad. It's funny cause the creator of Bravoman, who was a composer at Namco, says he got the idea from (musical) keyboards, which produce sounds of different intensity based on pressure, while those big SF buttons almost look like they wanted you to smash them, like those punching bag arcade games that measure how hard you punch. It's not impossible that they would have both come to the same idea from two different starting points.