Identifying compiler version from .NET binary
 

Is there a way to figure out what C# compiler version (not the .NET runtime version but the compiler version) was used to compile a binary or any other way to figure out how to recompile a decompiled .NET binary so it matches closer to the original binary when run through a tool like JustAssembly?

The .NET framework is your guide to determine which version of C# is used

refer to this : https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history

Check the answers here:
https://stackoverflow.com/questions/3460982/determine-net-framework-version-for-dll