Record types, immutable after construction: https://www.c-sharpcorner.com/article/c-sharp-9-0-introducti...
Warning when a switch isn't exhaustive: https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...
I don't believe C# has anything like the exhaustive switch over a sealed interface that the article is talking about.
OK, then I am going to regard this as "around the same time". Every new C# feature is released "in November", since .NET major releases drop in November every year. That's the release cadence.
But language design decisions of what to add in that release are not done inside that 1 release month, or in any 1 month. They are done over much longer time periods, often incrementally over successive years.