We use C# 9 in our WebForms monolith and it works fine. Just add `<LangVersion>latest</LangVersion>` to your .csproj file. (This assumes you're using recent Visual Studio version and use a recent version of MSBuild as well)
For example, to use record syntax, you need to define this attribute:
namespace System.Runtime.CompilerServices { public class IsExternalInit : Attribute { } }
https://docs.microsoft.com/en-us/dotnet/api/system.index?vie... https://docs.microsoft.com/en-us/dotnet/api/system.range?vie...
But pretty much all the other new C# language features I've tried have just worked straight out of the box with .NET 4.7.2.