It was a brief conversation in a social gathering for smallish engineer-led startups in my area. The one guy who went into details mentioned they are evaluating OpenBSD because of its presumed focus on security. I doubt they are super high throughput, but they might land on something else too. They run their own hardware at least, so the hardware compat issues of OpenBSD didn't seem to be a problem for them. FreeBSD was mentioned as well fwiw. I don't know how far along they got in performance testing or actual switch.

Just general angst some people were expressing for publicly exposed resources. The common wisdom just a few years ago was that throwing an internet facing nginx or haproxy on an up-to-date LTS linux with some firewall debouncer is as rock solid as you could get. And instead, you should worry about security in your own upstream app, not Linux's TCP sockets or nginx's regex path matching.