Via a plugin developed by some Microsoft employees:

https://mono-ue.github.io/

Yeah, the github.io page is still up, but the actual repo for that plugin has been deleted.

It's a private repo so you can't see it without Unreal Engine Github access, see my sibling comment for details.

That's either a dead project or it's moved somewhere that I can't find it.

Either way it's not something I'd trust enough to use for a commercial project, so I definitely wouldn't say UE supports C#.

No its still there. You just need to first get access to the Unreal Github repo to be able to see it since its a private repo (by signing up for an UE engine dev, it takes a couple minutes).

This is mentioned on the linked site:

"You will need source access to Unreal Engine on GitHub to get access to the MonoUE fork".

I did some digging around, I can't tell how active this project is, but given how little I found, this definitely isn't something I'd feel confident using outside of small experiments.

I wouldn't go around telling people UE4 supports C# based on this.