Stray bullets falling from altitude reach a terminal velocity which makes them only marginally lethal when they hit the ground.

Some people have been killed by falling bullets (more often from celebratory firing of guns in the air than from the rounds from aerial combat) but such incidents are rare, because falling bullets only have enough velocity and energy to be lethal in ideal conditions.

I believe I have seen prior questions on this topic on this sub-redditt which have given cases where death occurred, and which have detailed the terminal velocities and potential lethality of various types of ammunition falling from the skies. However, I could not find those threads.

The second wave of the Japanese attack on Pearl Harbor was met with a hail of anti-aircraft fire. Many of the anti-aircraft shells missed their intended targets and fell a good distance from the military bases around Pearl Harbor. 55 to 68 civilians in Honolulu were killed from errant shells falling on them. Now admittedly, a 127 millimeter shell has a lot more explosive power than a fifty caliber bullet or a 20 millimeter cannon shell, but it can be documented that civilians were killed by stray bullets in Honolulu on December 7th 1941
http://saturdaybriefing.outrigger.com/featured-post/pearl-harbor-attack-killed-a-lot-of-civilians-too/