Is it still in use? I thought it had been replaced by Kestrel.

Kestrel does ASP.Net very quickly, because that's all it does. If you want a full web server, that does things like static files, SSL, Reverse Proxying, etc. then you want IIS at the very least sitting in front of it.

See https://stackify.com/kestrel-web-server-asp-net-core-kestrel... - comparison table 3/4 of the way down.