Asset store: https://assetstore.unity.com/packages/tools/modeling/mesh-slicer-59618?aid=1011l8MKg

Mesh slicer plugin:
- Free test version
- Synchronous/Asynchronous slice;
- Slice meshes with colliders converting BoxCollider, SphereCollider and CapsuleCollider to MeshCollider;
- Skinned mesh support (beta version)
- Easy to extend;
- Very fast algorithm (just look comments on asset store);
- Smart optimization. So after slice you will not see any redundant triangles or vertex due to slice;
- Slice by plane;
- Slice by knife (examples).