The "backgroundkey" effect does nothing when I apply it on the timeline. I have a sample project that is too big to attach, but you can download it from https://dianne.skoll.ca/80638ba4901d7df88cb15d311d8cb2114f8afdcb/kdenlive-background-key.zip
I can confirm this behavior. See also https://discuss.kde.org/t/misc-backgroundkey-effect-how-is-it-supposed-to-work/33616