System.Text.Json

Added in .NET 5.0 (or Core 3.0).
The System.Text.Json library is included in the .NET Core 3.0 shared framework, while for other target frameworks, developers need to install the System.Text.Json NuGet package.

link - visualstudiomagazine.com/articles/2020/07/28/json-serializers.aspx 
link - learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to