Hi @clement,
Mesh.Split in Rhino 7 WIP is thread safe. Please note that there are better and more powerful overloads in Rhino WIP than the ones available on the online help for Rhino 6. Especially, we put a lot of thoughts into tolerance usage. See our new mesh intersection milestones page.
This is probably self-explanatory, but the fact that MeshSplit is thread safe does not mean that you can modify the mesh and at the same time split it, but it means that you can call various splits at the same time if the mesh in unchanged. Please be aware of memory usage piling up, especially if calling this in parallel on large meshes with large splitters.