Is there a System.Text.Json attribute to only allow valid enum values?

You can create a custom converter for your enum, like this:

using System;
using System.Text.Json;
using System.Text.Json.Serialization;

var jsonOpt = new JsonSerializerOptions();
jsonOpt.Converters.Add(new CustomEnumConverter());
Console.WriteLine(JsonSerializer.Deserialize<OrderInfo>("{\"TaxType\":14}", jsonOpt).TaxType);

public enum TaxType
{
    None = 0,
    GSTCanada = 5,
    HSTOntario = 13,
    HSTOther = 15
}

public class OrderInfo
{
    public TaxType TaxType { get; set; }
}

public class CustomEnumConverter : JsonConverter<TaxType>
{
    public override TaxType Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var value = reader.GetInt32();
        if (Enum.IsDefined(typeToConvert, value))
        {
            return (TaxType) value;
        }
        throw new Exception("Value is invalid");
    }

    public override void Write(Utf8JsonWriter writer, TaxType value, JsonSerializerOptions options)
    {
        writer.WriteNumber("taxType", (decimal)(int)value);
    }
}

Obviously this is not totally generic and you will need to do some work, but you get the idea.

Improvement on @MestreDosMagros's answer:

I created a [CheckedEnum] attribute.

It will throw an exception if the value is not in the enum.

You will need a different algorithm for a [Flags] enum.

Example:

public class OrderInfo
{
    public TaxType TaxType { get; set; }
}

[CheckedEnum]
public enum TaxType : byte
{
    None = 0,
    GSTCanada = 5,
    HSTOntario = 13,
    HSTOther = 15
}

(.NET fiddle tests)

Code:

public class CheckedEnumAttribute : JsonConverterAttribute { public CheckedEnumAttribute() : base(typeof(CheckedEnumConverterFactory)) { } }

public class CheckedEnumConverterFactory : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert)
        => typeToConvert.IsEnum;

    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
        => (JsonConverter)Activator.CreateInstance(typeof(CheckedEnumConverter<>).MakeGenericType(typeToConvert));
}

public class CheckedEnumConverter<T> : JsonConverter<T> where T: struct, Enum
{
    static readonly TypeCode typeCode = Type.GetTypeCode(typeof(T));

    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        T value = (T)(object)(typeCode switch
        {
            TypeCode.SByte => reader.GetSByte(),
            TypeCode.Byte => reader.GetByte(),
            TypeCode.Int16 => reader.GetInt16(),
            TypeCode.UInt16 => reader.GetUInt16(),
            TypeCode.Int32 => reader.GetInt32(),
            TypeCode.UInt32 => reader.GetUInt32(),
            TypeCode.Int64 => reader.GetInt64(),
            TypeCode.UInt64 => reader.GetUInt64()
        });
        if (!Enum.IsDefined(value)) throw new Exception($"Value {value} is invalid!");
        return value;
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        if (!Enum.IsDefined(value)) throw new Exception($"Value {value} is invalid!");
        if (typeCode == TypeCode.UInt64)
            writer.WriteNumberValue((ulong)(object)value);
        else
            writer.WriteNumberValue(Convert.ToInt64(value));
    }
}