Ironically, as I am creating a Visual Studio extension that migrates .NET 4.x code to .NET Core 3.1, I find myself mainly coding with .NET 4.7.2 and C# 7.3. While it is technically possible to use a subset of C# 8 features outside of .NET Core 3.x, the VS SDK is a difficult beast and introducing it to new things often makes it unhappy. I’ve lost an afternoon tracking down a reference to System.Collections.Immutable that was upsetting it.