[ComputerGuru]

To be clear, this has nothing to do with .NET Core 3.0 but rather is a feature of C# 8.0, which depends on compiler support that can be targeted to any previous version of .NET Core or Framework, all the way back to Framework 2.0 (tested myself, although MS says not to target 2.0 or 3.5).

[legulere]

C# 8 needs .net core 3.0 for some features to work at all like default interface method implementations, and for some to work meaningfully like nullable reference types.