I downloaded the oldest version of shotcut on their GitHub(14.05), but that gives me an error saying that I don’t have OpenGL 2.0.
I have an old laptop with a 2.7GHz Intel Celeron and 2 GB of RAM, this is the only free video editor I can find of that has chroma key but if anyone knows another one that will work on these specs that will be fine too.
You can also try older versions of kdenlive and openshot, both are free and open source like shotcut.
I also used to have a software called virtualdub, not very full fledged, but there was a third party plug in that allowed chroma key.
I will list some more softwares that you can use:-
Wax 2.0 by debugmode
Flowblade
Both are free, and flowblade seems to be constantly updated. But I think flowblade might lag, so for now I think openshot and kdenlive are the best, if shotcut does not work for you.