I would assume most of them? I'd be surprised if distros like Debian, Ubuntu, Fedora, etc. would ship non-mainline kernel features like that.

Sure, gaming-focused distros, or distros like Arch or Gentoo might (optionally or otherwise), but mainstream? Probably not.

Of course, esync doesn't require kernel patches, so I imagine that was more broadly out there. But it sounds like fsync got you performance pretty close to what ntsync can do, but esync was quite a bit behind both? With vanilla being quite a bit behind esync?

(Also, jeez, fsync, what a terrible name. fsync is a syscall that has to do with filesystem data. So confusing.)

Seriously, that was my thought too. Even if we were to stretch credibility and suggest that general consumers don't care about this sort of thing, they just released this for Windows Server in the past year?

Windows really is a toy of an OS. It continues to blow my mind that people want to use it as a server OS.