I worked at Yahoo Travel until 2011. When I left, transition to Linux was in full swing. If there's any FreeBSD left, the host count has got to be tiny by now. Running two OSes is a pain, and Linux has bigger mindshare, so there you go.

Vespa was extensively used for vertical search when I was there. Things like content search inside Travel, Local, Finance, Shopping, etc... was either Vespa or a predicessor. I don't think Vespa was used for the general internet search engine, but may have been used for some of the side queries that happen at the same time.

I don't have any insight into Yahoo search. Not sure about FreeBSD. My org is all on AWS + Azure

> What are the modern tech Stack behind Yahoo?

Too big to generalise. Shared tech stacks would be hard to call modern, but various tech stacks within teams were better. The more worthwhile stuff is open sourced (Athenz, Screwdriver) but there is a lot of shit internally, in varying stages of "deprecated but still the defacto solution".

> Still have any FreeBSD running?

I mean, there's probably some freebsd 7 hosts _somewhere_ and some of the internal tech stack supports it, but mostly it's RHEL6 when I left with a presumably-completed-by-now RHEL7 migration planned.

> Do Yahoo actually use Vespa [1] for their search engine?

Can't comment on search, but a partner team to my old team did.