IIRC at first they had an entire Microsoft stack (I may be mistaken on that).

But nowadays, from what I've read here on HN by SE devs in other threads, they're using lots and lots and lots of Linux: HAProxy, Redis, Nagios, etc.

I just double-checked the slide and although I didn't notice it at first, you can see that 'HA Proxy' and 'Redis' are mentioned.

The core Q&A is in C#/MS-SQL so that's probably not going to move to Linux anytime soon.