I would point to https://github.com/dotnet/roslyn itself as an example of a mode...

jmarolf on May 24, 2020 | parent | context | favorite | on: Welcome to C# 9.0

I would point to https://github.com/dotnet/roslyn itself as an example of a modern codebase. We try to adopt and use all of the new language features there as we write them to try and determine if the ergonomics are good.