I had no idea they used the Plan 9 file server for accessing files in WSL. I wonder what the original reasoning for choosing 9P was.

9P is in the kernel and provides a simple, fast networked VFS without many of the drawbacks of NFS. In particular, 9P-over-virtio is already used in Linux virtualization environments to expose parts of the host file systems to the guest. Since this solution was widely adopted and lying around, Microsoft's WSL2 solution also used 9P-over-virtio to expose the Windows file system to the guest Linux.