BSD

https://freebsdfoundation.org/netflix-case-study/

However https://lobste.rs/s/wh6yhk/why_we_run_freebsd_current_at_netflix has some more discussion.

Seems like BSD was the right answer when they started running a CDN. They developed a strong BSD team that contributes to upstream. At this point, retooling to use another OS is probably a massive investment for questionable gain (like, what would the benefits be?!).

Ultimately, they chose BSD, developed BSD, contributed to BSD.
At this point, it's a custom shoe they have helped design, and it fits like a glove. Or something