Idk about OP but where I live most public stalls do have indicators but not all of them. Often custom bathrooms for places like restaurants tend to be missing them since they’re to some degree custom designed to match a certain aesthetic and I’m guessing in those cases indicators are easily forgotten or just left out of more niche bathroom stall door designs.

I wouldn’t be surprised if this is also an issue in older bathrooms with older stall designs because I remember indicators being less common when I was a kid. But I usually knock if I’m checking a stall like that and don’t try to look underneath until I’ve tried less invasive measures. Honestly usually you can tell just by looking at the shadows on the floor and seeing if there’s any movement or not, without having to bend over to check for feet.