Frame difference mask What editing software for MacOS allows mask selection based on frame difference?

I have video footage taken from a tripod and I wish to create a tracking mask covering people and cars moving about the frame, using a frame without the people in it as reference.

Ideally I would like the software to ignore the objects that were already in the reference frame but moved away from it later (i.e., objects that are not part of the background).

Is this is a common feature in video-editing software or perhaps there is a plugin available for it?
The effect is known as a "difference" key. It's also known as a "not very good" key, because unless your footage is shot on a rock-solid locked off tripod and you're shooting uncompressed 4:4:4, it works a lot less well than you'd think.

After effects has a Colour Difference Key effect, as does Premiere, and this forum post explains how to do it in Apple Motion - though the principle will work in most compositing applications.

And of course, if you're after free as in speech as well as in beer Blender has a difference key node too.