Is it possible to create a plane on which only shadows from objects can be seen? Will shadows appear, for example, on a plane with 0 opacity?
Yes you can use a plane to only receive shadows.
I just discovered ShadowMaterial as well.
There are even more examples in the collection with links in the code.
For shadows see https://hofk.de/main/discourse.threejs/2017/Xindex2017.html