https://github.com/OxideMod/Oxide.CSharp/blob/develop/src/ObjectStream/Data/CompilerLanguageVersion.cs
I found out about this, tried changing it, building a new DLL and upload that. Then restarted my server. However it didn't do anything.
In the meantime I also did some research. Apparently Facepunch uses Unity 2019 LTS, which supports up to C# 7.3. Not sure if that applies to server and client, but I assume it does. If my assumption is correct, the server won't be able to run newer code anyway, would it? (https://docs.unity3d.com/2019.4/Documentation/Manual/CSharpCompiler.html)
If it was trivial to change, I would guess somebody would've already done it, or it would be done by default. Quite sure Wulf/Luke knows what he's doing, so there is probably a good reason why it's still compiling with target of C# 6.0. I did send him a message about it on uMod's website, however yet to hear back from him.