Since then, they migrated to .Net "Core"[0] which can run on any OS/web-server. Per [0] they initially kept IIS, but once you're on .Net Core+ that certainly isn't a requirement and there may be good licensing or performance reasons to migrate (even with headless Windows Server/IIS).

I did find an article from last year that said they still had a monolithic architecture and were still on-prem (as opposed to cloud/Azure). So, maybe, still on IIS? They certainly did for YEARS.

[0] https://www.infoq.com/news/2020/04/Stack-Overflow-New-Archit...