Why is Shutter Count important when purchasing a camera?
Every time we take a picture, the shutter, that sophisticated piece of engineering that exposes the sensor for a fraction of a second, wears out. After a large number of shutter releases, the wear will be such that the shutter will inevitably fail, leaving us with an unusable camera that requires an expensive shutter to work again.
Camera manufacturers know this and, through statistics, have calculated average shutter counts before equipment failure.
For this reason shutter count is a good indicator that gives us an idea of the wear and tear of a camera: imagine you have to choose between two cameras with similar looks and price, but one of them has 50k shutters and the other 1k; the decision would be obvious wouldn't it?
The first camera has some of its useful life travelled, while the second is just getting started.
Will a camera necessarily fail when it reaches its shutter count?
The shutter count is an average value, this does not mean that the camera will automatically fail when it reaches that number, but rather, it is what would be expected under normal conditions. Consider that there may be equipment that fails well before the number provided by the manufacturer and others that exceed it and continue to operate for a long time.
After this, it would seem that the shutter count is the only factor to take into account when purchasing equipment, but this is not the case. Evaluating a camera is very subjective and an art, rather than a science.
Do all cameras include shutter count in their metadata?
No, not all cameras include information, but some cameras do, regarding the shutter count of the equipment. That information is put in by the manufacturers and each one has different ways of doing it, so it is not a piece of information that is easily available to anyone, it involves an effort of reverse engineering.
Other less popular camera manufacturers may choose not to place that information in the metadata of their images to economize on their labor costs when manufacturing a camera.
For example most cell phones do not include this information, you can try and upload an image taken with one of your cell phones and see how it does not show any shutter count information.