I can only speak for my mid-2015 15" Retina MacBook Pro, and the whole pad is clearly flexible and very slightly sprung (and yes, it's made of glass - thin glass is quite flexible). You can see it flex and bend by looking across it while deflecting it - it's most easily seen by looking at the gap around the edge of the pad, between the pad and the laptop casing, while deflecting it close to the edge. The gap varies as the pad is deflected.
What it doesn't do is haptic feedback when it's turned off. There's no click. I infer that's because the click works by applying a mechanical kick back close to the point of deflection. Your finger wouldn't feel it as a click if there was no mechanical movement at all. A bit like a speaker: speakers are mechanical devices too.
> we’re pretty sure the magic pressure sensors in the new Force Touch trackpad are tiny strain gauges. Mounted on flexing metal supports, they detect the amount of flex on each—and based on that, the force from above.
