I’ve heard in Vegas Pro, you could do a shapeable mask in your source material and animate it. Why doesn’t Shotcut do that?
I’ve heard that with a Ferrari F40 you can go up to 200 mph
My Hyundai Kona can presumably only go to 120 mph
Why is that ? 
I am not the developer but i guess SC is simply not designed for that. In most cases you would use color masking as with a green or blue background - so this might be a solution for your needs?
In other cases you can use a 3d animation programm to draw and animate your characters and in the same run it would generate the according mask for you. I hardly can see any other need for an animatable mask shape. If the mask shape is steady but moves you can use the size&position filter instead to animate the mask movement (but not its shape - only its size).
LOL!
You can already do this with Mask: Simple Shape. Maybe you want to be able to draw an arbitrary shape, which is already requested multiple times and on the road map.