There is very little guidance regarding the Immutable specifier. The official documentation just says that it is only legal in the Object.h header file and is being phased out. It also notes that the specifier should not be used for new structs. Another comment from Class.h suggests that the struct uses binary serialization and that it is unsafe to add or remove members to the struct without incrementing the package version.
As of 4.25, the specifier was found in only twenty-one structs, all declared in the same file. It is best to avoid using this specifier going forward, as any existing support will likely terminate in future versions.