I believe the grid was removed from the ST-16 during the second firmware upgrade release. It is not difficult to mentally divide an image into thirds. Also, the rule of thirds is not a hard and fast rule, it's just a reference baseline for general photography.
Double tap the ST-16 screen while recording to see the image boundaries. Double tap again to return to the data screen. Bear in mind that when post processing a DNG file you can crop ~20% and not lose too much resolution. That 20% is about what you see on the screen when the data panels are visible. Cropping a JPG 20% costs a lot more resolution since they are much smaller files to begin with.