LOD is pretty straightforward to implement in Unity: Simply create a parent game object with a few child objects for the different distances attached as children, and have a script that, depending on the distance to the active camera enables/disables the renderers of those different object versions. You could also add complexity, e.g. depending on the number of objects currently visible or the current frame rate.

With voxels, I don't think this is possible in Unity at all, since AFAIK, voxels are a completely different approach for rendering (which simply isn't implemented in Unity). But maybe I'm missing something here (voxels combined with LOD might be a completely different story).

There's one forum posting regarding voxel based terrains in Unity: Voxel Based Terrain

And one pretty popular entry on Unity Feedback: Terrain: Voxel-based terrains

I am making a sandbox game and have done some research on voxels. Voxels can actually be used in Unity with the marching cubes algorithm. The platformer "IO" is a great example (This is not my game and I have no association with it or any of it's creators, I just found it on YouTube when looking for a way to use voxels). "IO" uses a system called Digital Clay, which allows smoothed voxels. If you are looking for a blockier, more minecraft-like voxel system, there are several asset packages that allow marching cubes voxels in unity. The asset store has very little for voxels, but a few searches for 'Unity voxels' or 'Unity marching cubes' will give you a few good results. One of the best voxel systems for Unity is called Voxelform, but it is paid-only and you need to contact the creator personally. I am going to be trying Digital Clay, but there are many other easy voxel systems for Unity. Of course, you could always just code your own, but the topic is very hard to understand and it is much easier to use open source.

In short, voxels can be implemented in Unity, and there are several assetpacks for it, but you can only do it with marching cubes.