How to do that like Sketchfab setting border fade in shadow ground plane

I try three-csm plugin but it’s useless for me

no fading:

set fading

Do you mean shadows like here ?

In this case, you can use soft shadows with a low-resolution shadow map. Or, if you want to get exactly the same effect as Sketchfab, try using SSAO post-processing with a very large kernel radius, which is what they are likely doing (it’ll also create some artsy artefacts, like the noisy background.)

no , you sample on sketchfab is setting by AO Baked

I wnat use shadow catcher and setting “border fade”

that difference between “AO Backed” and “Shadow catcher”

Contact shadows might be a good starting point for what you describe: three.js examples. The best result for a static model would to bake ambient occlusion in Blender and then export it to glTF, but this requires learning a bit of Blender usage.