• Polymorphism - covered by Update (and correct) the existing System.Text.Json article on polymorphism #30879.
• Utf8Reader.CopyString
• Json contract customization
• Json required properties (Implement a concept of "Required" properties runtime#29861)

See https://gist.github.com/krwq/a24138195a47fc248039314e8708dc40 for more info.