9P2000 is available on Linux with the CONFIG_9P_FS and related kernel compile options, which say things like:
"If you say Y here, you will get experimental support for Plan 9 resource sharing via the 9P2000 protocol. See http://v9fs.sf.net for more information."
It works in at least Ubuntu & Fedora out of the box, and is used by the virt-manager GUI to set up shares, so not too experimental to enable by default I guess.
On Arch Linux, it's been enabled by default for several years, and I've been using it daily for that time.
On x86_64, it's plenty stable, but I see why it's still experimental: On i686 there's a bug where files are mysteriously missing from directory listings. I tried to debug it once; it's reading them from the protocol correctly, but they're being dropped in the VFS layer, which is a bit more complicated than the network protocol.
"If you say Y here, you will get experimental support for Plan 9 resource sharing via the 9P2000 protocol. See http://v9fs.sf.net for more information."