There could be a simple 1 wall curved backdrop, a double wall and finally a three-wall seamless backdrop.
You should be able to add at least 2 backdrops to each model which you can drag, position, rotate and scale to fit it under your model's base and change the colour of the backdrop including with a colour picker.
With a pro Sketchfab account you should be allowed to put at least 20 to each model or maybe even more.
This feature would allow people to take screenshots of their models with shadows included which would place themselves along the backdrop when the model is positioned over it allowing even more proffesional images being taken in the screenshot tool. If you select "Transparent background (will reload)" it should remove the backdrop canvas or have a specific button to hide it encase you don't want the background but still want the canvas visible.