Tomaso offers a good solution in under 30 lines of code.
Object impact doesn't have to be complicated, as game developer Tomaso demonstrates in his showcase of mesh deformation in Unity. Projectiles leave visible indentations in just under 30 lines of code.
"A super simple mesh deformation in Unity using vertex offsets on impact with a smoothstep falloff. The deformation is computed off the main thread and applied once finished, keeping it fast and scalable even for large scenes and massive amounts of rigid bodies and collisions – especially when the MeshCollider is not updated."
Tomaso
You can grab the code on GitHub under SoftMeshLight. There are also some other interesting tools you should check out, including rope and cloth physics.