C# 9, but with some hacks you can bump it up to C# 10 - actually works and surprisingly stable. Can't wait for them to finally migrate to CoreCLR, though.

I got out of doing Unity development 7 years ago because I was tired of waiting for them to migrate to CoreCLR (among many other reasons).

Unfortunately the way they managed to stick with MonoRuntime and Burst, kind of made more harm than good, regarding C# adoption on the games industry.

Many issues people associate with C#, are actually only relevant in Unity, because of this.

Iirc the lineage of their c# came from Mono, then diverged a bit over time. Hopefully they can leave that baggage behind and just use the newer .net core, if they're not already that is... Disclaimer: I haven't looked in half a decade