JsonExtensionDataAttribute
When placed on a property of type IDictionary<T, U>, any properties that do not have a matching member are added to that dictionary during deserialization and written during serialization.
namespace System.Text.Json.Serialization
{
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
public sealed class JsonExtensionDataAttribute : JsonAttribute
{
}
}