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.)

Well I can tell you that if it didn’t make it upstream Fedora didn’t ship it.

It looks there was a copr for a custom kernel-fsync and projects like Bazzite or Nobara are adding patches.

From my understanding the fsync patches were never upstreamed.