With Unity/IL2CPP stuff: For general-purpose serialization libraries like JSON, you sometimes need to provide hints to make sure types are included: https://github.com/jilleJr/Newtonsoft.Json-for-Unity/wiki/Fi...

For schema serialization on known types, there are codegen tools (i.e. moc for MessagePack): https://github.com/neuecc/MessagePack-CSharp

MessagePack is migrating to Rosalyn code generators, so basically invisible codegen. Cysharp's newer serialization library, MessagePack, already uses this: https://github.com/Cysharp/MemoryPack