Since version 1903, Windows 10 can mount 9P network file systems. This is used from WSL. You can run, in Linux, "explorer.exe " and then see in Windows, a "network" (but local) share from your Linux instance.
Is this protocol open to use by other programs than WSL? I.e. could this new 9P client in Windows be used as an alternative to Dokan or WinFsp?
(Or is this 9P API internal to WSL and can only be originated from within WSL?)
9P is an open protocol. As of right now there is no possibility to configure client in Windows to enable third-party connections without some hacking.
https://en.wikipedia.org/wiki/9P_(protocol)