It's ancient. The latest version of Unity only partially supports C# 9. We're up to C# 14 now. But that's just the language version. The Mono runtime is only equivalent to .NET Framework 4.8 so all of the standard library improvements since .NET (Core) are missing. Not directly related to age but it's performance is also significantly worse than .NET. And Unity's garbage collector is worse than the default one in Mono.

The runtime is absolutely ancient, but I think the version number says more about C#'s churn than about how outdated the language version is. Take my opinion on C# with a grain of salt, though, I was an F#-er until the increasing interop pains forced me to drop it.