XML serialization is built into Noda Time itself, but other serialization mechanisms are expected to simply use the public API. (Noda Time 1.x and 2.x also support .NET binary serialization natively.)
Separating the code into a "core" in the nodatime repository and serialization code in a separate repository makes it simpler to implement independent release cycles.
All Noda Time serialization packages (as published by the Noda Time authors) start with a prefix "NodaTime.Serialization":
- NodaTime.Serialization.JsonNet (JSON serialization using Newtonsoft.Json)
- NodaTime.Serialization.SystemTextJson (JSON serialization using System.Text.Json)
- NodaTime.Serialization.Protobuf (Protocol Buffer serialization using Google.Protobuf)
The source code for all these packages is at https://github.org/nodatime/nodatime.serialization; issues should be filed in that GitHub repository.