Microsoft WSL2 uses plan9 filesystem
At least for the Ubuntu distribution in WSL2, the C:\ D:\ ... ntfs volumes in Windows are mounted in ubuntu side with mount -t drvfs
They must have implemented drvfs with plan9 source code because when you do df -Tm they show up as type 9p.
So parts of plan9 are used by millions of WSL users on Windows desktops.
Filesystem Type 1M-blocks Used Available Use% Mounted on
none tmpfs 12020 1 12020 1% /mnt/wsl
none 9p 476310 373837 102473 79% /usr/lib/wsl/drivers
/dev/sdc ext4 256947 83172 160652 35% /
none tmpfs 12020 26 11994 1% /mnt/wslg
none overlay 12020 0 12020 0% /usr/lib/wsl/lib
rootfs rootfs 12017 3 12015 1% /init
none tmpfs 12020 1 12020 1% /run
none tmpfs 12020 0 12020 0% /run/lock
none tmpfs 12020 0 12020 0% /run/shm
none tmpfs 12020 0 12020 0% /run/user
tmpfs tmpfs 12020 0 12020 0% /sys/fs/cgroup
none overlay 12020 1 12019 1% /mnt/wslg/versions.txt
none overlay 12020 1 12019 1% /mnt/wslg/doc
C:\ 9p 476310 373837 102473 79% /mnt/c
D:\ 9p 114471 517 113955 1% /mnt/d
F:\ 9p 2861589 1163060 1698529 41% /mnt/f
G:\ 9p 476310 378961 97349 80% /mnt/g
H:\ 9p 1826900 1029079 797821 57% /mnt/h
S:\ 9p 1826900 1029079 797821 57% /mnt/s
Y:\ 9p 2861589 5637 2855952 1% /mnt/y
Z:\ 9p 915188 224092 691097 25% /mnt/z