If you mean the ones that are just a painted strip on the ground, then OK, but for existing traffic lights controlled ones it doesn't seem unreasonable at all. Sensors to detect pedestrian and/or car presence are already extremely common. E.g. all modern [1] pedestrian crossings in the UK have similar sensors, which can extend the crossing time if it detects someone still in the road. And of course your local drive thru can detect your car pulling up, though usually by induction coil

[1] https://en.wikipedia.org/wiki/Puffin_crossing - standard since 2016

Not every crosswalk. This is explictly only for signaled intersections/crosswalks. i.e. those that already have a pushbutton, a walk sign, and a stoplight.

That only covers a very small portion of crosswalks and is generally done for crosswalks in high throughput areas or areas where risks of a pedestrian/vehicle collision are high. i.e. the places where you'd want additional augmentation to notify drivers and prevent collisions

Just make it precise enough to identify individuals, and some government secret agency will find a way to foot the bill.