Is it possible to use a ShaderMaterial vertex.glsl to select a subset of points that have a y position above a specific number?

I have a GLB file that I exported from Blender and have it imported into my ThreeJS scene. I have the GLB using a ShaderMaterial.

I would like to select a subset of points within the vertex.glsl that have a y position above a certain value (those in the box) so that I could have them animate in response to an event (load, click, unmount, etc).

Does anyone know of reference material, or strategies that show a way to select the subset?

Maybe you can use smoothstep function of glsl, I mean it will give you 0 or 1 if vertex is out of limit you provide and then you can use that value with uniforms to do something ig

It is possible. And the clip effect use this way(pass uniform value and do a judgement, then discard pixel in fragment shader)