I recall reading that Netflix chose FreeBSD a decade ago due to asynchronous disk IO was (and still is?) broken and/or limited to fixed block offsets. So nginx just works better on FreeBSD versus Linux for serving static files from spinning rust or SSD.

This used to be the case, but with io_uring, Linux has very much non-broken buffered async I/O. (Windows has copied io_uring pretty much verbatim now, but that's a different story.)

Could you expand more on the Windows io_uring bit please?

I have run Debian based Linux my entire life and recently moved circumstantially to Windows. I have no idea how it's kernel model works and I find io_uring exciting.

Wasn't aware of any adoption of io_uring ideas in Windows land, sounds interesting

Windows has had “IO completion ports” since the 1990s which work well and are high performance async for disk/network/other IO operations.

This isn't the same as the old Windows async I/O. ptrwis' links are what I thought of (and it's essentially a 1:1 copy of io_uring, as I understand it).