Plotly Mesh3d defines and displays an alpha-shape, but cannot return its simplicial structure.
To get it you can use pyvista (pip install pyvista), that can call the function Delaunay_3d with a prescribed
alpha (pyvista alpha = 1/alphahull; alphahull is an attribute of the Plotly Mesh3d). This function returns a mesh
and from mesh.cells and mesh.offset you can extract the simplices of dimension 0, 1, 2, 3 that define the simplicial structure of the alpha shape. In this case you can plot the corresponding alpha shape as a Mesh3d from points and triangles (i.e. providing x, y, z, i, j, k to define a an instance of go.Mesh3d). Example https://plot.ly/~empet/15549/.