- [x] [Polymorphism](https://devblogs.microsoft.com/dotnet/announcing-dotnet-7-preview-5/#polymorphism) - covered by https://github.com/dotnet/docs/issues/30879. - [x] [Utf8Reader.CopyString](https://devblogs.microsoft.com/dotnet/announcing-dotnet-7-preview-5/#utf8jsonreader-copystring) - [x] [Json contract customization](https://devblogs.microsoft.com/dotnet/announcing-dotnet-7-preview-6/#json-contract-customization) - [x] Json required properties (https://github.com/dotnet/runtime/issues/29861) See https://gist.github.com/krwq/a24138195a47fc248039314e8708dc40 for more info.
System.Text.Jsonarticle on polymorphism #30879.See https://gist.github.com/krwq/a24138195a47fc248039314e8708dc40 for more info.