Has there been any movement on this? This would be an awesome feature we would love to have as well.

To resurrect this one, I just had an issue where after we upgraded a boolean token from a query was going in as 'True' instead of 'true' when used in workflow web request JSON, which caused it to start failing every time.

It was an easy fix by converting it to lowercase, but this is another example of when a JSON format option could be beneficial; I'd imagine it working somewhat similar to JSON.Encode in ASP.NET web pages.

Jason, there are two different standard JSON encodings in the .NET Framework 4.8.1 System.Text.Encodings.Web JavaScriptEncoder class that you can use with System.Text.Json JsonEncodedText.Encode method:

1. Default

2. UnsafeRelaxedJsonEscaping

   1. See also: Serialize all characters, especially the "Caution" note

I wrote a quick test program to compare the two with your sample string and a boolean. The Encode() method require string input, and .NET's Boolean.ToString() method returns "True" or "False", so that doesn't help you with the ToLower scenario.

The JsonSerializer.Serialize method returns 'true' as you pass it the Boolean directly without having to first cast it to a string.

What behavior would you expect here?

Output:

Code for reference:

// .NET Framework 4.8.1
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.Json;
using System.Text.Encodings.Web;
using System.Threading.Tasks;

namespace JsonEncoderDemo
{
    internal class JsonEncoder
    {
        static void Main()
        {
            string inputStr = "X\"Y\\Z";
            bool inputBool = true;

            JsonSerializerOptions jsoDefault = new JsonSerializerOptions() { Encoder = JavaScriptEncoder.Default };
            JsonSerializerOptions jsoUnsafeRelaxed = new JsonSerializerOptions() { Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping };

            string outputStrEncoderDefault = JsonEncodedText.Encode(inputStr, JavaScriptEncoder.Default).ToString();
            string outputStrEncoderUnsafeRelaxed = JsonEncodedText.Encode(inputStr, JavaScriptEncoder.UnsafeRelaxedJsonEscaping).ToString();
            var outputStrSerializedDefault = JsonSerializer.Serialize(inputStr, jsoDefault);
            var outputStrSerializedUnsafeRelaxed = JsonSerializer.Serialize(inputStr, jsoUnsafeRelaxed);

            string outputBoolEncoderDefault = JsonEncodedText.Encode(inputBool.ToString(), JavaScriptEncoder.Default).ToString();
            string outputBoolEncoderUnsafeRelaxed = JsonEncodedText.Encode(inputBool.ToString(), JavaScriptEncoder.UnsafeRelaxedJsonEscaping).ToString();
            var outputBoolSerializedDefault = JsonSerializer.Serialize(inputBool, jsoDefault);
            var outputBoolSerializedUnsafeRelaxed = JsonSerializer.Serialize(inputBool, jsoUnsafeRelaxed);

            Console.WriteLine("Input String:         " + inputStr);
            Console.WriteLine("Encoded (Default):    " + outputStrEncoderDefault);
            Console.WriteLine("Encoded (Relaxed):    " + outputStrEncoderUnsafeRelaxed);
            Console.WriteLine("Serialized (Default): " + outputStrSerializedDefault);
            Console.WriteLine("Serialized (Relaxed): " + outputStrSerializedUnsafeRelaxed);
            Console.WriteLine("");
            Console.WriteLine("Input Bool:           " + inputBool);
            Console.WriteLine("Encoded (Default):    " + outputBoolEncoderDefault);
            Console.WriteLine("Encoded (Relaxed):    " + outputBoolEncoderUnsafeRelaxed);
            Console.WriteLine("Serialized (Default): " + outputBoolSerializedDefault);
            Console.WriteLine("Serialized (Relaxed): " + outputBoolSerializedUnsafeRelaxed);
            Console.WriteLine("");
        }
    }
}

Some good related discussion on Stack Overflow here: 
c# - dotnet core System.Text.Json unescape unicode string - Stack Overflow