This blog article shows the resources for C# 9.0. Whether it is available in .NET Framework, officially I understand it is, no. People do claim it works. It is time for you to find out.
What’s new in C# 9.0
C# language versioning
Take your first steps with C#
Call methods from the .NET Class Library using C#
.NET Tutorial – Hello World in 10 minutes
Record Types In C# 9